tests/posix/common: Adjust miscalibrated timing test

This test seems a little confused.  It does a POSIX usleep() for 90ms,
then checks the time taken, and verifies that it was no less
than... 91ms!

On existing platforms, tick alignment makes sure that we always take a
little longer, so this passes. But on high tick rate configurations we
get it exactly right.  And fail.

Adjust the calibration to allow (exactly) 90ms sleeps.  Also fixed a
comment that described the wrong units.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>