selftests/timers: Improve skew_consistency by testing with other clockids

Lei Chen reported a bug with CLOCK_MONOTONIC_COARSE having inconsistencies
when NTP is adjusting the clock frequency.

This has gone seemingly undetected for ~15 years, illustrating a clear gap
in our testing.

The skew_consistency test is intended to catch this sort of problem, but
was focused on only evaluating CLOCK_MONOTONIC, and thus missed the problem
on CLOCK_MONOTONIC_COARSE.

So adjust the test to run with all clockids for 60 seconds each instead of
10 minutes with just CLOCK_MONOTONIC.

Reported-by: Lei Chen <lei.chen@smartx.com>
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250320200306.1712599-2-jstultz@google.com
Closes: https://lore.kernel.org/lkml/20250310030004.3705801-1-lei.chen@smartx.com/