Fix to unit test losing process output due to timing issue

[JuhPuur]

Description

The loop would read lines until the child process terminated, but if the process terminated before every line was read we would lose output and the testing person might have concluded that no tests were run after that point. This fixes that by reading the remnant after the child process terminates.

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

[ciarmcom]

@JuhPuur, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[cmonr]

CI started

[mbed-ci]

Test run: FAILED

Summary: 1 of 1 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_unittests

[0xc0170]

Please review the failure [linux] TypeError: __init__() got an unexpected keyword argument 'encoding'

[JuhPuur]

Fixed it. I was testing with python 3.7 and forgot to revert. subprocess module has that argument in later versions.

[0xc0170]

CI restarted

[mbed-ci]

Test run: SUCCESS

Summary: 9 of 9 test jobs passed
Build number : 2
Build artifacts