meson: Optionally disable installation of test modules

One open issue (IMO) with the meson build system is that it installs the
test modules under src/test/modules/ as part of a normal installation.
This is because there is no way to set up up the build system to install
extra things only when told. I think we still need a way to disable
this somehow, so that building a production installation doesn't end up
with a bunch of extra files.

The attached simple patch is a starting point for discussion. It just
disables the subdirectory src/test/modules/ based on some Boolean
setting. This could be some new top-level option, or maybe part of
PG_TEST_EXTRA, or something else? With this, I get an identical set of
installed files from meson. I imagine this option would be false by
default and developers would enable it.

Thoughts?