gettime: add basic init cpuclock test

Some systems claim to have a synced and constant rate tsc, even if
they don't. So add the basic cpuclock-test test as part of fio
trusting the CPU clock, to avoid using the CPU clock on a system
where the TSC truly isn't synced between CPUs.

Signed-off-by: Jens Axboe <axboe@fb.com>