Run tests of libpq on installcheck-world, checkprep and check-world

Run tests of libpq on installcheck-world, checkprep and check-world

The regression tests of libpq have been converted to a TAP test as of
ac25173, but they were not run when running the global make targets
mentioned in $subject as the code only considered src/interfaces/ecpg/
as a path to run.  This switches to src/interfaces/, to always include
the tests of libpq in each run.

Reported-by: Justin Pryzby
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/20220529151850.GN19626@telsasoft.com