CI-test the performance

[kvark]

Some of the incoming changes affect the performance. This is sometimes being caught by the servo test bots, since the slowness opens up a bunch of intermittent issues that seem to already be there. Unfortunately, this only happens when servo wants to update the WR dependency, and is annoying.

@glennw suggested to run the sample under OSMesa driver.

[nical]

I think that this is now done in https://wrperf.org/