fix a bug where AdvanceNext unlocks twice (#10)

Co-authored-by: edwingeng <edwingeng@outlook.com>

fix: fix swallowing ticks on TickerFunc (#8)

Turns out #7 is bugged.  When we refused to call the TickerFunc callback, we didn't advance the next event time, so the Mock kept re-scheduling `fire()` and we could never finish processing on the tick time.

fix: fix mock TickerFunc to act like real (#7)

fixes #5
fixes #6

Fixes the mock TickerFunc to behave like the real one in that it won't run more than one callback at a time and will wait until the callback completes to return from a `Wait()` call.

chore: add CCO license

Signed-off-by: Spike Curtis <spike@coder.com>