Convert src/interfaces/libpq/test to a tap test.

The old form of the test needed a bunch of custom infrastructure. These days
tap tests provide the necessary infrastructure to do better.

We discussed whether to move this test to src/test/modules, alongside
libpq_pipeline, but concluded that the opposite direction would be
better. libpq_pipeline will be moved at a later date, once the buildfarm and
msvc build infrastructure is ready for it.

The invocation of the tap test will be added in the next commit. It involves
just enough buildsystem changes to be worth commiting separately. Can't happen
the other way round because prove errors out when invoked without tests.

Discussion: https://postgr.es/m/20220223203031.ezrd73ohvjgfksow@alap3.anarazel.de

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