[testrunner] Fix test selection on Windows

This makes flako usable on Windows using standard test names.
A workaround to bisecting to failures before this CL is to replace the
test_name by something like "mjsunit/regress\regress-1138075", i.e.
using this on the command line to trigger flako:
-p 'test_name="mjsunit/regress\\regress-1138075"'

R=liviurau@chromium.org

Bug: v8:9218
Change-Id: I37596efcaeca780eeacb27c2841fe8302ddb1e49
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3081610
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Liviu Rau <liviurau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76222}

tools/testrunner/testproc/filter.py

@@ -4,6 +4,7 @@
 
 from collections import defaultdict
 import fnmatch
+import os
 
 from . import base
 
@@ -80,4 +81,9 @@ class NameFilterProc(base.TestProcFilter):
       if fnmatch.fnmatch(test.path, g):
         return False
     exact_matches = self._exact_matches.get(test.suite.name, {})
-    return test.path not in exact_matches
+    if test.path in exact_matches: return False
+    if os.sep != '/':
+      unix_path = test.path.replace(os.sep, '/')
+      if unix_path in exact_matches: return False
+    # Filter out everything else.
+    return True