Fix flaky DiagnosticTests.Session_TracksActivities by waiting for full server activity predicate #2652

Job	Run time
build (macos-latest, Debug)	11m 29s
build (windows-latest, Debug)	14m 57s
build (ubuntu-latest, Release)	10m 39s
build (macos-latest, Release)	9m 27s
build (ubuntu-latest, Debug)	10m 46s
build (windows-latest, Release)	16m 2s
publish-coverage / publish-coverage	31s
	1h 13m 51s

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