[crash][feedback] switch to RunLoopUntil() in unit tests

* apparently RunLoopWithTimeoutUntil() is bad because the default timeout
isn't guaranteed to be enough on some bots so instead we rely on the
infra timeout they give to each test
* we don't check the return value RunLoopUntil() in anticipation of https://fuchsia-review.googlesource.com/c/fuchsia/+/280471

TESTED=`fx run-test feedback_agent_tests`
TESTED=`fx run-test crashpad_agent_tests`

Change-Id: I491252ef6a3e3f51f418ef5e57d3f426411e54f6