Make captured stdout and stderr available within the subunit stream

With the planned change from the fix to bug 996729 in place, we (will) have no access to the stdout and stderr in test runs on buildbot (or anywhere subunit is used). As a next step, we should make it available. I propose that these should be reported as faux tests, just like we do with layer information. Add tags (one for stdout and one for stderr) that we can filter on.

Since this will presumably be done by the yellow squad in support of the parallel tests, please also fix lpbuildbot to filter out these from the test count *and* include new logs of just this output, so people can easily look at them. Nice to have: the name of the worker that generated the stdout or stderr output. Nice to have: anytime stdout or stderr is generated, the log also writes the name of the test that is listed for that worker that is run immediately after any stdout or stderr output. If you'd like to make a separate bug/card for this, great. If it is easy and you want to combine them, that's fine too.