tests/thread_apis: Allow 20µs slop in k_busy_wait test

When testing k_busy_wait(100), don't require that at least 100µs of CPU
time be consumed by the thread itself; some of that time may be used
elsewhere in the system. Instead, just make sure that at least 80µs is
accounted to the thread and allow 20µs to get used elsewhere.

Signed-off-by: Keith Packard <keithp@keithp.com>