VTN Coordinator test may fail if too many threads are present.

I observed that VTN Jenkins job failed due to failure of VTN Coordinator UT.

https://jenkins.opendaylight.org/vtn/job/vtn-verify/628/

[ RUN ] atomic.sub_old_uint8_race
test_atomic.cc:631: Failure
Expected: (0U) != (c), actual: 0 vs 0
[ FAILED ] atomic.sub_old_uint8_race (270 ms)

This test creates 4 threads, and let them invoke atomic operation.
Although the test expects all test threads invoked atomic operation at least once,
it may not invoke if the thread scheduler on the build host did not dispatch
test threads. This test needs to ensure that all test threads has been dispatched
at least once.