tests: latency_measure: reduce the chance of cycles underflow

Sometimes there's an unusually large cycles for tests that are
known to complete with just a few cycles. Upon some testing,
I found that it was because the overhead cycles was larger
than the cycles taken by tests, causing the cycles to
underflow.

To workaround that, make sure that the overhead measurement
thread runs uninterrupted, repeat the measurement for a few
times, and take the minimum value.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>