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/expected/pg_stat_statements.out

@@ -0,0 +1,21 @@
+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();
+ pg_stat_statements_reset 
+--------------------------
+ 
+(1 row)
+
+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;
+                       query                        | calls | rows 
+----------------------------------------------------+-------+------
+ SELECT pg_stat_statements_reset();                 |     1 |    1
+ UPDATE test SET b = ? WHERE a > ?;                 |     1 |    5
+ INSERT INTO test VALUES(generate_series(?, ?), ?); |     1 |   10
+(3 rows)
+
+DROP TABLE test;
+DROP EXTENSION pg_stat_statements;