Add minimal set of regression tests for pg_stat_statements.

Andres Freund <andres@anarazel.de>

Commit: 9be244dbe876038676e92e34d0f9fe55914376d2
Author: Andres Freund <andres@anarazel.de>
Date: 2016-11-12T13:01:48Z
Releases: 10.0
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>

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