Add minimal set of regression tests for pg_stat_statements.

While the set of covered functionality is fairly small, the added tests
still are useful to get some basic buildfarm testing of
pg_stat_statements itself, but also to exercise the lwlock tranch code
on the buildfarm.

Author: Amit Kapila, slightly editorialized by me
Reviewed-By: Ashutosh Sharma, Andres Freund
Discussion: <CAA4eK1JOjkdXYtHxh=2aDK4VgDtN-LNGKY_YqX0N=YEvuzQVWg@mail.gmail.com>

contrib/pg_stat_statements/sql/pg_stat_statements.sql

@@ -0,0 +1,15 @@
+CREATE EXTENSION pg_stat_statements;
+
+CREATE TABLE test (a int, b char(20));
+
+-- test the basic functionality of pg_stat_statements
+SELECT pg_stat_statements_reset();
+
+INSERT INTO test VALUES(generate_series(1, 10), 'aaa');
+UPDATE test SET b = 'bbb' WHERE a > 5;
+
+SELECT query, calls, rows from pg_stat_statements ORDER BY rows;
+
+DROP TABLE test;
+
+DROP EXTENSION pg_stat_statements;