tests: kernel: threads: Correct test assertion in thread_apis test

Thread APIs test for k_busy_wait incorrectly asserted that a delay of
100 us should produce a delay in cycles less than or equal to to 100 cycles
of the hardware clock. Since most hardware clocks are fast, this assertion
was valid, but it does not test for the actual delay.

Fix the assertion to verify that a delay of 100 us produces a delay in
cycles less than or equal to 100 us worth of hardware clock cycles.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>