ut: fd test will fail if we shuffle tests

[vagetablechicken]

rdsn/src/failure_detector/tests/failure_detector.cpp:464: Failure
Value of: spin_wait_condition([&wait_count]() -> bool { return wait_count == 0; }, 20)
  Actual: false
Expected: true
[  FAILED  ] fd.old_master_died (20002 ms)
[----------] 6 tests from fd (80018 ms total)

[----------] Global test environment tear-down
[==========] 6 tests from 1 test case ran. (80018 ms total)
[  PASSED  ] 4 tests.
[  FAILED  ] 2 tests, listed below:
[  FAILED  ] fd.worker_died_when_switch_master
[  FAILED  ] fd.old_master_died