run:<x> command always returns exit code of 0

[powli]

Preconditions

1. Any supported Magento version
2. Any MFTF version

Steps to reproduce

Run the vendor/bin/mftf run:test, vendor/bin/mftf run:group or vendor/bin/mftf run:failed command with failing tests

Expected result

The command should return a non-zero exit code as errors occured

Actual result

The command returns an exit code of zero

This is especially relevant in a CI environment where many job setups depend on the exit code of the run commands. In this case, we had a job showing as green even though the MFTF tests in the job failed.
This also blocks command chaining in the CLI, e.g. vendor/bin/mftf run:test FailingTest >> test.log && echo "Test success" will always print out "Test success".

[powli]

Resolved with #338