Only keep logs for failing tests

[fzipi]

What if we can only keep the logs for failed tests? That way it would simplify downloading artifacts on the CRS test pipeline.

One way could be:

• run a test
• if it goes good, zero the error log file
• if there is a failure, concatenate it to a new file "failed-tests.log" for example
• repeat for every test
• in the end, there should be an empty error log, and maybe a file "failed-tests.log" if something failed.
• probably it would make sense only if the flag --show-failures-only is enabled.