powCluster.js process still alive after Duniter stop

@nanocryk already raised this issue, and I could also observe this behavior with the powCluster.ts unit test with the following code:

"use strict";

const co = require('co');
const should = require('should');
const PowEngine = require('../../../app/modules/prover/lib/engine').PowEngine
const logger = require('../../../app/lib/logger').NewLogger()

describe('PoW Engine', () => {

  it('should be configurable', () => co(function*(){
    const e1 = new PowEngine({ nbCores: 1 }, logger);
    (yield e1.setConf({ cpu: 0.2, prefix: '34' })).should.deepEqual({ cpu: 0.2, prefix: '34' });
    const e2 = new PowEngine({ nbCores: 1 }, logger);
    yield e1.shutDown()
  }));

  it('should be able to make a proof', () => co(function*(){
    const e1 = new PowEngine({ nbCores: 1 }, logger);
    const block = { number: 35 };
    const zeros = 2;
    const highMark = 'A';
    const pair = {
      pub: 'HgTTJLAQ5sqfknMq7yLPZbehtuLSsKj9CxWN7k8QvYJd',
      sec: '51w4fEShBk1jCMauWu4mLpmDVfHksKmWcygpxriqCEZizbtERA6de4STKRkQBpxmMUwsKXRjSzuQ8ECwmqN1u2DP'
    };
    const forcedTime = 1;
    const medianTimeBlocks = 20;
    const avgGenTime = 5 * 60;
    const proof = yield e1.prove({
        newPoW: {
          block,
          zeros,
          highMark,
          pair,
          forcedTime,
          conf: {
            medianTimeBlocks,
            avgGenTime
          }
        }
      }
    )
    proof.should.deepEqual({
      pow: {
        block: {
          number: 35,
          time: 1,
          inner_hash: '51937F1192447A96537D10968689F4F48859E2DD6F8F9E8DE1006C9697C6C940',
          nonce: 212,
          hash: '009A52E6E2E4EA7DE950A2DA673114FA55B070EBE350D75FF0C62C6AAE9A37E5',
          signature: 'bkmLGX7LNVkuOUMc+/HT6fXJajQtR5uk87fetIntMbGRZjychzu0whl5+AOOGlf+ilp/ara5UK6ppxyPcJIJAg=='
        },
        testsCount: 211,
        pow: '009A52E6E2E4EA7DE950A2DA673114FA55B070EBE350D75FF0C62C6AAE9A37E5'
      }
    });
    yield e1.shutDown()
  }));

  it.only('should be able to stop a proof', () => co(function*(){
    const e1 = new PowEngine({ nbCores: 1 }, logger);
    // yield e1.forceInit()
    const block = { number: 26 };
    const zeros = 10; // Requires hundreds of thousands of tries probably
    const highMark = 'A';
    const pair = {
      pub: 'HgTTJLAQ5sqfknMq7yLPZbehtuLSsKj9CxWN7k8QvYJd',
      sec: '51w4fEShBk1jCMauWu4mLpmDVfHksKmWcygpxriqCEZizbtERA6de4STKRkQBpxmMUwsKXRjSzuQ8ECwmqN1u2DP'
    };
    const forcedTime = 1;
    const medianTimeBlocks = 20;
    const avgGenTime = 5 * 60;
    const proofPromise = e1.prove({
        newPoW: {
          block,
          zeros,
          highMark,
          pair,
          forcedTime,
          conf: {
            medianTimeBlocks,
            avgGenTime
          }
        }
      }
    )
    // yield new Promise((res) => setTimeout(res, 10))
    // yield e1.cancel()
    // const proof = yield proofPromise;
    // should.not.exist(proof);
    yield e1.shutDown()
  }));
});

Then if I launch the test:

 yarn tsc && node --trace-warnings node_modules/mocha/bin/_mocha --grep "PoW Engine"

I can see in my console that, even if the test is done, I still get messages of another process:

(node:19136) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): undefined
    at emitWarning (internal/process/promises.js:63:21)
    at emitPendingUnhandledRejections (internal/process/promises.js:95:11)
    at runMicrotasksCallback (internal/process/next_tick.js:124:9)
    at _combinedTickCallback (internal/process/next_tick.js:131:7)
    at process._tickCallback (internal/process/next_tick.js:180:9)
(node:19136) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
    at emitWarning (internal/process/promises.js:78:15)
    at emitPendingUnhandledRejections (internal/process/promises.js:95:11)
    at runMicrotasksCallback (internal/process/next_tick.js:124:9)
    at _combinedTickCallback (internal/process/next_tick.js:131:7)
    at process._tickCallback (internal/process/next_tick.js:180:9)
(node:19136) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 2): undefined
    at emitWarning (internal/process/promises.js:63:21)
    at emitPendingUnhandledRejections (internal/process/promises.js:95:11)
    at runMicrotasksCallback (internal/process/next_tick.js:124:9)
    at _combinedTickCallback (internal/process/next_tick.js:131:7)
    at process._tickCallback (internal/process/next_tick.js:180:9)
(node:19136) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 3): undefined
    at emitWarning (internal/process/promises.js:63:21)
    at emitPendingUnhandledRejections (internal/process/promises.js:95:11)
    at runMicrotasksCallback (internal/process/next_tick.js:124:9)
    at _combinedTickCallback (internal/process/next_tick.js:131:7)
    at process._tickCallback (internal/process/next_tick.js:180:9)
(node:19136) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 4): undefined
    at emitWarning (internal/process/promises.js:63:21)
    at emitPendingUnhandledRejections (internal/process/promises.js:95:11)
    at runMicrotasksCallback (internal/process/next_tick.js:124:9)
    at _combinedTickCallback (internal/process/next_tick.js:131:7)
    at process._tickCallback (internal/process/next_tick.js:180:9)

This is a problem: the sub-processes launched by Duniter (here, the tests) should be killed along with Duniter being stopped.