all-tests: exclude passthru attributes from test discovery

discoverTests tries to discover some sort of internal function and tries to call
it with the arguments for that internal function. This poses an issue when you
want to expose some other functions (i.e. a parameterisation for a test) in
nixosTests.

This commit allows a test to pass through arbitrary values via `.passthru`
without them having discovery applied to them; including functions.

nixos/tests/all-tests.nix

@@ -21,7 +21,7 @@ let
     if isAttrs val
     then
       if hasAttr "test" val then callTest val
-      else mapAttrs (n: s: discoverTests s) val
+      else mapAttrs (n: s: if n == "passthru" then s else discoverTests s) val
     else if isFunction val
       then
         # Tests based on make-test-python.nix will return the second lambda