Remove the direct use of the microsecond and low power ticker from
the tests. This enforces that sleep mode is properly locked when
using timers. Furthermore, this prepares the codebase for new ticker
changes which allow differing clock frequencies and timer bit widths.
The lp_ticker test overrides the default ticker handler for the
low power ticker. This stops all other low power TimerEvents in the
system, including the ones for tickless, from getting called. Because
of this devices with tickless enabled malfunction during this test.
This patch fixes this problem by passing all lp ticker events it did
not trigger on to the TimerEvent irq handler.