tests: posix: clock: Workaround to align clocks before querying

Before calling clock_gettime() 2 times in row, issue k_usleep(1)
to align code execution to timer interrupt to prevent (well,
minimize) possibility of getting different ticks values.

Suggested by @andyross.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>