Fix potential test race conditions.

A few of the tests start threads and rather than wait for the
thread to get to a particular point, the test just sleeps and
hopes for the best. Change these tests to actually wait until
the threads are running before proceeding.

Bug: 417996715

Test: All unit tests pass.
Test: Ran the flaky tests 1000 times on oriole and on cf_x86_64_foldable-trunk_staging-userdebug.
Change-Id: I0914e23ab95c1d30a89680d17f8af46b736439c4