Enhance performance timing tests for esr45
We currently have tests for making sure the Navigation Timing and Resource Timing APIs are disabled. We should enhance these tests to make sure
a) the getEntries methods are not working to get timing information back (done in the resource timing test)
b) the Performance Observer (https://bugzilla.mozilla.org/show_bug.cgi?id=1165796) does not provide a way to get this data
c) write a corresponding test for the User Timing API (we had #16336 (moved) for disabling the preference).
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