esp_timer: fix occasional failures in "monotonic values" test

esp_timer: fix occasional failures in "monotonic values" test

1. ref_clock used in unit tests occasionally produces time off by ~100
microseconds shortly after being started. Add a delay to let
ref_clock stabilise, until the cause is found.

2. Reduce roundoff error accumulation which would occasionally cause
the test to fail, by choosing an overflow value which can be divided
by APB frequency.

3. Move time sampling part of the test into an IRAM function to
reduce variations due to cache behavior.

4. Remove calculation of "standard deviation" in the test, as what was
calculated was not actually standard deviation, and it did not add any
useful information.