The test targets libpq's options, so 'src/test/interfaces/libpq/t' is
a more natural place for it.
While doing this, I noticed that I had missed adding the
libpq_encryption subdir to the Makefile. That's why this commit only
needs to remove it from the meson.build file.
Per Peter Eisentraut's suggestion.
Discussion: https://www.postgresql.org/message-id/09d4bf5d-d0fa-4c66-a1d7-5ec757609646@eisentraut.org
