Add test for slow-running callback

This test makes a tick callback run for longer than the update interval
by busy-waiting.  The purpose of having it is that the stopwatch should
not rely on being able to get an interval event for each and every
interval.  Instead, it should use the amount of elapsed time as
measured by the system clock.
Add test where stopwatch is paused and restarted

This test is to verify that the stopwatch's time does NOT increase
while the stopwatch is in a "stopped" state.  The codebase already
passes the test as of this commit, but when it is switched to actually
use the system clock to get the elapsed time, this becomes important
to verify.
Use wall clock for reporting elapsed time