postgresql: refactor postgresqlVersions attribute & tests

Every postgresql testcase essentially does the following things:

* Filter `postgresqlVersions` for server packages
* Filter postgresql server packages for suitable ones (i.e. extensions
  must support the given version)
* Generate an attribute-set of testcases

The first item became necessary in
7ab1e88833 given that
`postgresql/default.nix` now exposes JIT and non-JIT servers AND a
`libpq` that is not suitable for the tests here.

This changes restructures this a little bit, i.e.:

* Having an attribute-set that contains a bunch of postgresql servers
  and a single client package seems odd (and the sole consumer of
  `postgresqlVersions` in nixpkgs, the test suite, has to take that into
  account). Hence, postgresql's default.nix now provides `libpq` (the client)
  and a `postgresqlVersions` attribute with all supported JIT and non-JIT
  variants of postgresql.

* Each test-case gets a third argument, a function called `genTests`:
  this function sets `recurseForDerivations = true;` and generates an
  attribute-set of tests for each postgresql version given a function
  that returns a testcase or multiple test-cases (`makeTestFor`). The
  argument to `makeTestFor` is a postgresql server package.

  This function also accepts a filter predicate that is passed against
  `filterAttrs` to remove postgresql server packages that are not
  suitable for the test (e.g. because the version isn't supported by the
  extension to test).

I checked by making sure that the `.drv` doesn't change on staging with
this change on top for postgresq, postgresql-jit,
postgresql-wal-receiver, postgresql-tls-client-cert, anonymizer, pgjwt,
pgvecto-rs, timescaledb, tsja and wal2json.

nixos/tests/postgresql/anonymizer.nix

@@ -1,6 +1,7 @@
 {
   pkgs,
   makeTest,
+  genTests,
 }:
 
 let
@@ -107,11 +108,7 @@ let
       '';
     };
 in
-lib.recurseIntoAttrs (
-  lib.concatMapAttrs (n: p: { ${n} = makeTestFor p; }) (
-    lib.filterAttrs (_: p: p ? pkgs && !p.pkgs.anonymizer.meta.broken) pkgs.postgresqlVersions
-  )
-  // {
-    passthru.override = p: makeTestFor p;
-  }
-)
+genTests {
+  inherit makeTestFor;
+  filter = _: p: !p.pkgs.anonymizer.meta.broken;
+}