Improving test coverage of extensions with pg_dump

Hi all,

While investigating the issue that has been committed as ebd092b to
fix FK dependencies in pg_dump for tables in extensions, I developed a
small test case aimed for integration in src/test/modules to ensure
that this does not break again in the future. Note that as the
regression tests of this test module use TAP tests to run a set of
pg_dump commands and check the sanity of FK dependency tracking with
extensions, this has needed a one-line tweak of the target prove_check
to be able to install the contents of the current directory to the
temp install folder before running the tests.

I imagine that it would be nice to integrate this test case to improve
test coverage of pg_dump, extending at the same time TAP support for
extensions, so attached are patches for this purpose.

Those patches are really simple, but then perhaps there are better or
simpler ways than what is attached, so feel free to comment if you
have any ideas.
Regards,
--
Michael