Introduce useUnconfinedTestDispatcher, etc, use them

This is the first step in a process that will allow us to use
UnconfinedTestDispatcher in most tests, reducing the need to
use runCurrent except where needed for precise tests of
concurrent logic.

Details at go/thetiger

Bug: 342622417
Test: local unit test runs of changed tests
Flag: TEST_ONLY

Change-Id: Iabc6b609e97109aa28fe4534fb7a0a14c1082cf2