Added function to set cycle time in ms

Added and implemented the SetTimerCycleTimeMilliSec function as well as
the system-related test mocks it relies on. Also fixed argument ordering
in test assertions and redefined register mocks as proper 8-bit unsigned
integers.
Refactored to have SetTimerCycleTimeMilliSec deal with frequencies instead of prescaler factors.
Began SetTimerCycleTimeSec implementation.