Support space and backslash characters in runfile filenames.

Bazel supports them with bazelbuild/bazel#23331.

Step 1: support them for directory-based runfiles.  This doesn’t require any
code changes, just adapt the test.

elisp/runfiles/runfiles-test.el

@@ -39,14 +39,15 @@
 (ert-deftest elisp/runfiles/special-chars/directory ()
   (let ((directory (make-temp-file "runfiles-test-" :directory)))
     (unwind-protect
-        (let ((filename (expand-file-name "testäα𝐴🐈'.txt" directory))
+        (let ((filename (expand-file-name "testäα𝐴🐈' \t\n\\.txt" directory))
               (runfiles (elisp/runfiles/make :manifest "/invalid.manifest"
                                              :directory directory))
               (coding-system-for-write 'utf-8-unix)
               (write-region-annotate-functions nil)
               (write-region-post-annotation-function nil))
           (write-region "contents\n" nil filename nil nil nil 'excl)
-          (should (equal (elisp/runfiles/rlocation "testäα𝐴🐈'.txt" runfiles)
+          (should (equal (elisp/runfiles/rlocation "testäα𝐴🐈' \t\n\\.txt"
+                                                   runfiles)
                          filename)))
       (delete-directory directory :recursive))))