Add regression test coverage for contrib/pg_buffercache.

Add regression test coverage for contrib/pg_buffercache.

We can't check the output of this view very closely without
creating portability headaches, but we can make sure that
the number of rows is as-expected.  In any case, this is
sufficient to exercise all the C code within, which is a
lot better than the 0% coverage we had before.

DongWook Lee

Discussion: https://postgr.es/m/CAAcByaLCHGJB7qAENEcx9D09UL=w4ma+yijwF_-1MSqQZ9wK6Q@mail.gmail.com