meson: Add missing dependencies for libpq tests

meson: Add missing dependencies for libpq tests

The missing dependency was, e.g., visible when doing
  ninja clean && ninja meson-test-prereq && meson test --no-rebuild --suite setup --suite libpq

This is a bit more complicated than other related fixes, because until now
libpq's tests depended on 'frontend_code', which includes a dependency on
fe_utils, which in turns on libpq. That in turn required
src/interfaces/libpq/test to be entered from the top-level, not from
libpq/meson.build.  Because of that the test definitions in libpq/meson.build
could not declare a dependency on the binaries defined in
libpq/test/meson.build.

To fix this, this commit creates frontend_no_fe_utils_code, which allows us to
recurse into libpq/test from withing libpq/meson.build.

Apply this to all branches with meson support, as part of an effort to fix
incorrect test dependencies that can lead to test failures.

Discussion: https://postgr.es/m/CAGECzQSvM3iSDmjF+=Kof5an6jN8UbkP_4cKKT9w6GZavmb5yQ@mail.gmail.com
Discussion: https://postgr.es/m/bdba588f-69a9-4f3e-9b95-62d07210a32e@eisentraut.org
Backpatch: 16-, where meson support was added