[test] Simplify output processor logic

Instead of conditionally normalizing file paths, go back to using
the basename in the dynamically-generated regular expression
pattern.

Bug: v8:7950, v8:9395, v8:9406
Change-Id: I57d2449a8229a67b038c7fcffe36218848d9a575
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1681122Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62439}

tools/testrunner/outproc/message.py

@@ -37,13 +37,10 @@ class OutProc(base.OutProc):
     # the extension in the first place.
     base_path = self._basepath + '.js'
     if not os.path.exists(base_path):
-      # d8's output for module tests contains forward slashes for
-      # file paths as opposed to backslashes on Windows. Address this
-      # by normalizing only in this specific case.
-      base_path = self._basepath.replace('\\', '/') + '.mjs'
+      base_path = self._basepath + '.mjs'
 
     env = {
-      'basename': base_path,
+      'basename': os.path.basename(base_path),
     }
     for (expected, actual) in itertools.izip_longest(
         expected_lines, actual_lines, fillvalue=''):