runner: Capture runner output in runner_test instead of spewing it on screen

runner_test invokes the runner internally a few times and multiple of those invocation spew a lot of messages on stderr/stdout.

Spotting which subtest has failed due to which assert is hard when we get multiple screens worth of expected errors/--help messages.

All of that extra noise should be captured on the side and printed out only when relevant/unexpected.

Example: https://gitlab.freedesktop.org/gfx-ci/igt-ci-tags/-/jobs/1025808
Actual error: TIMEOUT 30.02 s