tests: posix: clock: do not use usleep in a broken way

Using `usleep()` for >= 10000000 microseconds results
in an error, so this test was kind of defective, having
explicitly called `usleep()` for seconds.

Also, check the return values of `clock_gettime()`.

Signed-off-by: Chris Friedt <cfriedt@meta.com>
(cherry picked from commit 23a1f0a6)