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

src/test/isolation/specs/stats.spec

@@ -6,10 +6,13 @@ setup
     INSERT INTO test_stat_tab(key, value) VALUES('k0', 1);
     INSERT INTO test_stat_oid(name, oid) VALUES('test_stat_tab', 'test_stat_tab'::regclass);
 
-    CREATE FUNCTION test_stat_func() RETURNS VOID LANGUAGE plpgsql AS $$BEGIN END;$$;
+    -- include 10us sleep to ensure that runtime measures as more than zero
+    CREATE FUNCTION test_stat_func() RETURNS VOID LANGUAGE plpgsql AS
+      $$BEGIN PERFORM pg_sleep(10e-6); END;$$;
     INSERT INTO test_stat_oid(name, oid) VALUES('test_stat_func', 'test_stat_func'::regproc);
 
-    CREATE FUNCTION test_stat_func2() RETURNS VOID LANGUAGE plpgsql AS $$BEGIN END;$$;
+    CREATE FUNCTION test_stat_func2() RETURNS VOID LANGUAGE plpgsql AS
+      $$BEGIN PERFORM pg_sleep(10e-6); END;$$;
     INSERT INTO test_stat_oid(name, oid) VALUES('test_stat_func2', 'test_stat_func2'::regproc);
 
     CREATE TABLE test_slru_stats(slru TEXT, stat TEXT, value INT);