Tweak a failing custom test

* test/lisp/custom-tests.el (custom-tests-require-theme): Fix a
native-comp-related test failure.

In this test, we set load-path to nil, and under native-comp some of
the tests want to give a warning:

Warning (comp): Cannot look-up eln file as no source file was
found for /tmp/custom-tests-bfdbkW/custom-tests--d.elc

But we can't load warning.el after setting load-path to nil, so
require it first.

test/lisp/custom-tests.el

@@ -96,6 +96,7 @@ Ensure the directory is recursively deleted after the fact."
 
 (ert-deftest custom-tests-require-theme ()
   "Test `require-theme'."
+  (require 'warnings)
   (custom-tests--with-temp-dir
     (let* ((default-directory temporary-file-directory)
            (custom-theme-load-path (list default-directory))