Don't hardcode contrib_regression dbname in postgres_fdw and dblink tests.

That allows parallel installcheck to succeed inside contrib/. The
output is not particularly pretty unless make's -O option to
synchronize the output is used.

There's other tests, outside contrib, that use a hardcoded,
non-unique, database name. Those prohibit paralell installcheck to be
used across more directories; but that's something for a separate
patch.