Fix typo in Ticker example

Implement reset for mocked tickers

The "too late!" version of this call was being made *after* the
corresponding test had completed, causing all manner of sadness from the
testing package.

This refactors the tests to just measure the duration of the various
kinds of sleep, and assert that the duration is within a good range.

Fix tests to not call t.Fatal from a goroutine

This defers execution of the ticked function until after the lock has
been released, allowing it to make other calls without a deadlock.

Address re-entrancy of Tick

add a Duration type alias

Using a mock for time doesn't mean races aren't important.  This tries
to set a good Example by using channels rather than shared variables for
communication between goroutines.

Co-authored-by: Josh Soref <[email protected]>

add Until

Use channels to avoid races in tests