Fix delay tests

[edeandrea]

Fix unit tests introduced by #31 which can now be exercised by #34.

[edeandrea]

Stopwatch.elapsed(TimeUnit) loses precision if there are fractional units. From the Stopwatch.elapsed(TimeUnit) docs:

Returns the current elapsed time shown on this stopwatch, expressed in the desired time unit, with any fraction rounded down.

Note: the overhead of measurement can be more than a microsecond, so it is generally not useful to specify TimeUnit.NANOSECONDS precision here.

It is generally not a good idea to use an ambiguous, unitless long to represent elapsed time. Therefore, we recommend using elapsed() instead, which returns a strongly-typed Duration instance.