Move test runner logger to `mozlog.proxy.QueuedProxyLogger`?

[gsnedders]

We currently rely on our own log messages:

wpt/tools/wptrunner/wptrunner/testrunner.py

Lines 197 to 203 in f1c69d8

	# Ensure that when we start this in a new process we have the global lock
	# in the logging module unlocked
	release_mozlog_lock()
	proc_name = mpcontext.get_context().current_process().name
	logger = structuredlog.StructuredLogger(proc_name)
	logger.add_handler(LogMessageHandler(send_message))

However, mozlog actually provides its own multiprocessing logger support which we could use:

wpt/tools/third_party_modified/mozlog/mozlog/proxy.py

Lines 40 to 46 in f1c69d8

	class QueuedProxyLogger(StructuredLogger):
	"""Logger that logs via a queue.
	This is intended for multiprocessing use cases where there are
	some subprocesses which want to share a log handler with the main thread,
	without the overhead of having a multiprocessing lock for all logger
	access."""

This was added by @jgraham back in 2017: mozilla-firefox/firefox@995f6ab, and used in the server from 7c178d2 until 8a2f868.

I don't know if there was any reason why the test runner itself never used this, rather reimplementing it itself?