Add minimal sleep to stats isolation test functions.

Add minimal sleep to stats isolation test functions.

The functions test_stat_func() and test_stat_func2() had empty
function bodies, so that they took very little time to run.  This made
it possible that on machines with relatively low timer resolution the
functions could return before the clock advanced, making the test fail
(as seen on buildfarm members fruitcrow and hamerkop).

To avoid that, pg_sleep for 10us during the functions.  As far as we
can tell, all current hardware has clock resolution much less than
that.  (The current implementation of pg_sleep will round it up to
1ms anyway, but someday that might get improved.)

Author: Michael Banck <mbanck@gmx.net>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/68d413a3.a70a0220.24c74c.8be9@mx.google.com
Backpatch-through: 15