test-org-attach-git: Fix recent failure to load tests

* testing/lisp/test-org-attach-git.el: Provide feature.

As of 9044b300e (org-test: Fix ERC errors using latest Emacs master,
2021-11-24), tests must provide a feature.

testing/lisp/test-org-attach-git.el

@@ -92,4 +92,5 @@
 	   (should (string-equal "hello world\n" (buffer-string))))
 	 (should (eq called 'was-called)))))))
 
+(provide 'test-org-attach-git)
 ;;; test-org-attach-git.el ends here