install our custom timestamp formats in a less disruptive way

install our custom timestamp formats in a less disruptive way
The unit tests on Windows fail because trial is attempting to remove its own
log observer during teardown.  This patch customizes the extant log observer
object by replacing its formatTime method with our own.

I first tried the approach of storing their log observer object and putting it
back during teardown, but it didn't work (perhaps because our node object
doesn't get a chance to do its deferred stopService behavior in time), and
anyway I generally prefer the "fail-safe", or "crash-only" design.