timers: backport f8193ab.

Original commit message:

 timers: use uv_now instead of Date.now

 This saves a few calls to gettimeofday which can be expensive, and
 potentially subject to clock drift. Instead use the loop time which
 uses hrtime internally.

In addition to the backport, this commit:
 - keeps _idleStart timers' property which is still set to
   Date.now() to avoid breaking existing code that uses it, even if
   its use is discouraged.
 - adds automated tests. These tests use a specific branch of
   libfaketime that hasn't been submitted upstream yet. libfaketime
   is git cloned if needed when running automated tests.