libpq_pipeline in tmp_install

The test program libpq_pipeline produced by the test suite in
src/test/modules/libpq_pipeline/ is installed into tmp_install as part
of make check. This isn't a real problem by itself, but I think it
creates a bit of an asymmetric situation that might be worth cleaning up.

Before, the contents of tmp_install exactly matched an actual
installation. There were no extra test programs installed.

Also, the test suite code doesn't actually use that installed version,
so it's not of any use, and it creates confusion about which copy is in use.

The reason this is there is that the test suite uses PGXS to build the
test program, and so things get installed automatically. I suggest that
we should either write out the build system by hand to avoid this, or
maybe extend PGXS to support building programs but not installing them.
The advantage of the former approach is that it would allow additional
test programs to be added later as well. (We should really collect the
libpq tests under src/interfaces/libpq/ anyway at some point.)