Fix flaky DiagnosticTests.Session_TracksActivities by waiting for full server activity predicate #2624

Job	Run time
build (ubuntu-latest, Release)	12m 53s
build (windows-latest, Debug)	11m 50s
build (windows-latest, Release)	12m 50s
build (macos-latest, Debug)	6m 33s
build (ubuntu-latest, Debug)	13m 3s
build (macos-latest, Release)	6m 42s
publish-coverage	0s
	1h 3m 51s

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