Use unconfined test dispatcher in ZenModeInteractorTest

There's a chance this might fix some test flakiness, but even if it
doesn't, this is the recommended dispatcher to use for tests with simple
coroutines like this one.

Bug: 387006867
Flag: TEST_ONLY
Test: this
Change-Id: I14d359029ded643fa3a459fffbd9f8f5ac684776