Reland "[test] Fix cctest path separators on Windows"

test/test262/testcfg.py

Index: test/test262/testcfg.py
diff --git a/test/test262/testcfg.py b/test/test262/testcfg.py
index a0c2cd6568232524ad00742ec81eaa91bf1c3ef1..835c1ffd0b0ff80de800556ccfa1973c6c9408d9 100644
--- a/test/test262/testcfg.py
+++ b/test/test262/testcfg.py
@@ -66,8 +66,9 @@ class Test262TestSuite(testsuite.TestSuite):
       files.sort()
       for filename in files:
         if filename.endswith(".js"):
-          testname = os.path.join(dirname[len(self.testroot) + 1:],
-                                  filename[:-3])
+          fullpath = os.path.join(dirname, filename)
+          relpath = fullpath[len(self.testroot) + 1 : -3]
+          testname = relpath.replace(os.path.sep, "/")
           case = testcase.TestCase(self, testname)
           tests.append(case)
     return tests