nixos/testing: fix testScript eval for functions without ...

The nspawn container support (23f1e6370d) added a `containers`
argument to the testScript caller. This breaks tests whose testScript
uses strict pattern matching without ellipsis, e.g. `{ nodes }:`,
since Nix rejects unexpected arguments.

Use `builtins.intersectAttrs` to only pass arguments the function
actually expects, making the caller forward-compatible with future
argument additions.

Fixes: 23f1e6370d ("nixos/test-driver: add support for nspawn containers")

nixos/lib/testing/testScript.nix

@@ -14,6 +14,13 @@ in
   options = {
     testScript = mkOption {
       type = either str (functionTo str);
+      apply =
+        v:
+        if lib.isFunction v then
+          # Only pass args the testScript function expects.
+          args: v (builtins.intersectAttrs (lib.functionArgs v) args)
+        else
+          v;
       description = ''
         A series of python declarations and statements that you write to perform
         the test.