meson: Prevent installation of test files during main install

meson: Prevent installation of test files during main install

Previously, meson installed modules under src/test/modules/ as part of
a normal installation, even though these files are only meant for use
by tests.  This is because there is no way to set up up the build
system to install extra things only when told.

This patch fixes that with a workaround: We don't install these
modules as part of meson install, but we create a new "test" that runs
before the real tests whose action it is to install these files.  The
installation is done by manual copies using a small helper script.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/2a039e8e-f31f-31e8-afe7-bab3130ad2de%40enterprisedb.com