Arithmetic underflow in balance transfer integration test

An integration test added in 7f9f2d27 reveals a bug.

To replicate:

# 1. checkout this commit
# 2. run the test
cargo test -p gdev-runtime test_simple_transfer 
# 3. this is the output
running 1 test
test test_simple_transfer ... FAILED

failures:

---- test_simple_transfer stdout ----
thread 'test_simple_transfer' panicked at runtime/gdev/tests/integration_tests.rs:1098:9:
Expected Ok(_). Got Err(
    DispatchErrorWithPostInfo {
        post_info: PostDispatchInfo {
            actual_weight: None,
            pays_fee: Pays::Yes,
        },
        error: Arithmetic(
            Underflow,
        ),
    },
)
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    test_simple_transfer

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 24 filtered out; finished in 1.64s

(this bug was already here before !182 (merged))