make check for PLs

Here is a patch that implements make check support for the PLs and a
make check-world target.

I found the lack of this quite annoying while reviewing the load of
PL/Python patches. There did not appear to be a reason why it wasn't
done before.

Support for make check in contrib would also be nice, but it's currently
not possible without a way to teach pg_regress to install the respective
module's shared library.