Add assertion to avoid deadlocks in Task.join #539

Job	Run time
Dub Tests (ubuntu-latest, dmd-latest, x86_64)	3m 26s
Dub Tests (ubuntu-latest, ldc-latest, x86_64)	3m 40s
Dub Tests (windows-latest, dmd-latest, x86_64)	3m 58s
Dub Tests (windows-latest, ldc-latest, x86_64)	4m 55s
Dub Tests (macOS-13, dmd-latest, x86_64)	9m 14s
Dub Tests (macOS-13, ldc-latest, x86_64)	10m 17s
Dub Tests (ubuntu-latest, dmd-2.096.1, x86_64)	3m 28s
Dub Tests (windows-latest, dmd-2.092.0, x86_64)	4m 1s
Dub Tests (windows-latest, dmd-2.092.0, x86_mscoff)	4m 15s
Dub Tests (windows-latest, dmd-2.091.1, x86_64)	4m 4s
Dub Tests (windows-latest, ldc-1.20.1, x86_64)	4m 49s
	56m 7s

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