let pytest capture kernel output

[minrk]

Now that we're using pytest, we can let it handle capturing kernel output. If we run kernels for tests with extra_arguments=['--log-level=10'], stdout=sys.stdout, stderr=sys.stdterr, when a test fails, it will report debug-level logging from the process if/when the test fails. This can often be helpful for debugging failures, where currently we suppress all output unconditionally so all we have is the failure condition, without any logs to help figure out why it failed.