Use TREBLE_TESTING_OVERRIDE only in FMQ unit test client.

Usage of the env variable in the FMQ unit test service
was causing oneway methods to execute in a separate
thread which was causing timing issues and resultant
flakiness for some test cases.

Bug: 63825555
Test: Ran fmq_test 1000 times on Marlin
Change-Id: I9f752181e6fba0ef8f82216e513261c0861ef90e
2 files changed