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?