[test] [fix] #368 : wrong use of context `with pytest.raises() as...`

  * correct the form of tests

tests/test_unit_tx.py

@@ -192,9 +192,9 @@ def test_compute_amounts_errors(capsys):
                 trial[0],
                 trial[1],
             )
-            expected_error = "Error: amount {0} is too low.".format(trial[0][0])
-            assert capsys.readouterr() == expected_error
         assert pytest_exit.type == SystemExit
+        expected_error = "Error: amount {0} is too low.".format(trial[0][0])
+        assert capsys.readouterr().out == expected_error
 
 
 def test_compute_amounts():
@@ -1344,6 +1344,7 @@ def test_check_transaction_values_errors(
         result = tx.check_transaction_values(
             comment, outputAddresses, outputBackChange, enough_source, issuer_pubkey
         )
+    assert pytest_exit.type == SystemExit
     display = capsys.readouterr()
     if comment.find("Wrong_Char_") != -1:
         assert display == "Error: the format of the comment is invalid"
@@ -1353,19 +1354,13 @@ def test_check_transaction_values_errors(
         assert display.out.find("Error: bad format for following public key:") != -1
     elif outputBackChange:
         if outputBackChange == "Wrong_Pubkey":
-                assert (
-                    display.out.find("Error: bad format for following public key:")
-                    != -1
-                )
+            assert display.out.find("Error: bad format for following public key:") != -1
     elif enough_source is True:
         assert (
-                display.out.find(
-                    "pubkey doesn’t have enough money for this transaction."
-                )
+            display.out.find("pubkey doesn’t have enough money for this transaction.")
             != -1
         )
     assert result == ""
-    assert pytest_exit.type == SystemExit
 
 
 # test generate_unlocks()