Await for the gecko profiler start instead of adding a synthetic delay

Feature/improvement

Currently browsertime adds a delay after the gecko profiler start command:
https://github.com/sitespeedio/browsertime/blob/79e9b0e1f72c3005641b1131173cdaaf24f93a51/lib/firefox/geckoProfiler.js#L89-L90
This was because previously we didn't have a way to properly await for the profiler start since all the processes start the profiler individually. But this year, with the Bug 1668867, we've started to return a Promise instead from the Services.profiler.StartProfiler API and it resolves when all the processes start profiling. This way we can start the profiler and await until it's ready without needing an additional settle delay.

Here's the profiler start command in browsertime:
https://github.com/sitespeedio/browsertime/blob/79e9b0e1f72c3005641b1131173cdaaf24f93a51/lib/firefox/geckoProfiler.js#L74-L77

Here's an example in our tests: https://searchfox.org/mozilla-central/rev/0a2eba79c24300ce0539f91c1bebac2e75264e58/tools/profiler/tests/xpcshell/test_active_configuration.js#24-30

Start in lib/firefox/geckoProfiler.js at the profiler start command and the following delay around lines 74-90. Compare the implementation with the linked Mozilla xpcshell test for Services.profiler.StartProfiler. Done means the profiler startup is awaited and the synthetic settle delay is no longer needed, with relevant tests passing.