performance-timeline/po-observe.html should not hardcode entry types

[samweinig]

The test performance-timeline/po-observe.html currently hardcodes the list of entry types it needs to observe (see https://github.com/web-platform-tests/wpt/blob/master/performance-timeline/po-observe.html#L81) but really should be filtering that list by PerformanceObserver.supportedEntryTypes so that the test limits itself to only testing performance-timeline defined things and is not reliant on the implementation having implemented other standards not strictly needed to test the functionality.

[sideshowbarker]

dunno who’s responsible for the Performance Timeline test suite these days, but maybe @plh @caribouW3 can help