Steady thread condition wait for OSX

For Linux, the celix functions `celixThreadCondition_waitUntil`, `celixThreadCondition_timedwaitRelative`, `celixThreadCondition_getTime` and `celixThreadCondition_getDelayedTime` are based on a monotonic time to prevent unsteady behavior due to OS sleeps or time updates.
This is possible because the `celixThreadCondition_init` function initializes the underlining thread condition with time MONOTONIC.

For OSX this is not possible.
Currently for OSX a relative sleep is calculated and a call to `pthread_cond_timedwait_relative_np` is used.
Based on test result this seems unsteady, but further investigation is needed. There is also not a lot of documentation for `pthread_cond_timedwait_relative_np`. Does it even return a ETIMEDOUT code?

If this is updated the following test can be reenabled:
ScheduledEventTestSuite, ScheduledEventTimeoutLogTest

And the error margin in ScheduledEventTestSuite and ThreadsTestSuite can be made stricter.