Deflake tests that get flakier when getEntity stops being guarded by a
transaction.
DeletePropagationTest: add a read barrier to guarantee write has completed
before reading in
foreignReference_writtenForeignReference_isReadBackAsSameForeignReference
HandlesTestBase: Poll when dereferencing to guarantee underlying changes
get picked up.
Deflake tests that get flakier when getEntity stops being guarded by a transaction.
DeletePropagationTest: add a read barrier to guarantee write has completed before reading in foreignReference_writtenForeignReference_isReadBackAsSameForeignReference
HandlesTestBase: Poll when dereferencing to guarantee underlying changes get picked up.