Make test suite finish faster

[sarsko]

Made the test suite faster. The semaphore changes are quite interesting, as the only thing happening there is that we use the main thread for one of the tasks instead of spawning, which drastically cuts down on the number of schedules.

Slowest tests: before vs. after

#	Before	Time	After	Time
1	mpsc_some_senders_with_blocking	77.8s	async_mpsc_some_senders_with_blocking	22.7s
2	async_mpsc_some_senders_with_blocking	52.2s	dropped_acquire_must_release_random (fair)	5.8s
3	batch_semaphore_test_2	34.5s	dropped_acquire_must_release_random (unfair)	5.6s
4	semtest_2	30.6s	ui (trybuild)	5.0s
5	async_mpsc_many_senders_with_blocking	30.2s	clock_condvar_notify_all_dfs	3.2s
6	runtime_mpsc_many_senders_with_blocking	25.0s	mpsc_recv_from_outside_runtime	3.0s
	Sum of top 6	250.3s	Sum of top 6	45.3s

Slowest single test: 77.8s → 22.7s (~3.4x faster). Combined top-6 time: 250.3s → 45.3s (~5.5x faster).
The mpsc/semaphore tests that dominated the old tail are gone from the top 6 entirely.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jorajeev]

Generally looks fine, though for non-dfs tests, we really don't have a great way to know "how many iterations are enough"