Enabled VCL Timer unittests on Windows builds and added two unittests to
detect misfiring of timer events.

Due to the nature of timer events having virtually no guarantees on
non-realtime operating systems, the tests may fail randomly under
different system loads.  The tests repeat a few times in such cases, but
still there are no guarantees.

These tests are added to detect gross breaks in the timer algorithms and
to improve certain corner cases.  They are designed to minimize any
random failures.

Posterity might hate me for committing such unittests, so I added
TEST_TIMERPRECISION directive to include/exclude the tests at compile
time. It should be enabled if it causes more harm than good.

Change-Id: If2856f194cb4732c84900113bdb969f397f67d5a
Reviewed-on: https://gerrit.libreoffice.org/17906Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>