Implement xTimerGetExpiryTime() and xTimerIsTimerActive()

[NeroBurner]

In InfiniTimeOrg/InfiniTime#494 (comment) @Riksu9000 informed me that the linked PR uses two functions, that are not yet implemented in the simulator:

@NeroBurner The simulator doesn't have these two functions xTimerGetExpiryTime() and xTimerIsTimerActive(). If this ever gets merged, they will need to be added.