convert libpq uri-regress tests to tap test

Hi,

When verifying that the meson port actually runs all perl based tests I came
across src/interfaces/libpq/test/regress.pl. Instead of running tests yet
another way, it seems better to convert it to a tap test.

I hope others agree?

Where would we want that test to live? Right now we have the slightly odd
convention that some tap tests live in src/test/{misc,modules,...}. But
e.g. frontend binary ones are below src/bin/.

For now I've left it in src/interfaces/libpq/test, with the test in
t/001_uri.pl. But we should at least get rid of the test/...

Perhaps we should just rename src/test/modules/libpq_pipeline to
src/test/modules/libpq and move uri-regress in there as well?

The tap test needs a bit more polish, mostly posted this to get some feedback
before wasting more time :)

Greetings,

Andres Freund