check_stability.py is too verbose

[foolip]

After #10988 it looks like the stability jobs are more verbose. https://travis-ci.org/web-platform-tests/wpt/jobs/389217237 (for #11301) is an example of this, where the raw log ends with "Log reached capacity (3145728 bytes); output disabled" during "Repetition 8 / 10".

The job failed, and it's not possible to tell why.

[foolip]

This affected #11333 too. There the log ended like this:

Log reached capacity (3145728 bytes); output disabled.


No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
Check the details on how to adjust your build configuration on: https://docs.travis-ci.com/user/common-build-problems/#Build-times-out-because-no-output-was-received

The build has been terminated

And only in "Repetition 4 / 10", so the verbosity has to be at least halved to fix this.