meson: Add 'running' test setup, as a replacement for installcheck

meson: Add 'running' test setup, as a replacement for installcheck

To run all tests that support running against existing server:
$ meson test --setup running

To run just the main pg_regress tests against existing server:
$ meson test --setup running regress-running/regress

To ensure the 'running' setup continues to work, test it as part of the
freebsd CI task.

Discussion: https://postgr.es/m/CAH2-Wz=XDQcmLoo7RR_i6FKQdDmcyb9q5gStnfuuQXrOGhB2sQ@mail.gmail.com