lib/tests/release.nix: temporary reference to pkgs/test/release

This commit temporarily adds pkgs/test/release to the
lib/tests/release.nix test suite, because ofborg already knows about
that entry point.

We should move the list of test entry points out of ofborg and into
a central place in nixpkgs:

  https://github.com/NixOS/nixpkgs/issues/272591

Once we do that we won't need to have this ugly kludge in an
inappropriate place.

lib/tests/release.nix

@@ -67,5 +67,17 @@ let
 in
   pkgs.symlinkJoin {
     name = "nixpkgs-lib-tests";
-    paths = map testWithNix nixVersions;
+    paths = map testWithNix nixVersions ++
+
+      #
+      # TEMPORARY MIGRATION MECHANISM
+      #
+      # This comment and the expression which follows it should be
+      # removed as part of resolving this issue:
+      #
+      #   https://github.com/NixOS/nixpkgs/issues/272591
+      #
+      [(import ../../pkgs/test/release {})]
+    ;
+
   }