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

test/mjsunit/testcfg.py

Index: test/mjsunit/testcfg.py
diff --git a/test/mjsunit/testcfg.py b/test/mjsunit/testcfg.py
index 4fbdcfca26626d7867e1d920a9832e382f5fe642..7af7acf0a927e2ac034aa459cd4aaa796618ea14 100644
--- a/test/mjsunit/testcfg.py
+++ b/test/mjsunit/testcfg.py
@@ -52,7 +52,9 @@ class MjsunitTestSuite(testsuite.TestSuite):
       files.sort()
       for filename in files:
         if filename.endswith(".js") and filename != "mjsunit.js":
-          testname = os.path.join(dirname[len(self.root) + 1:], filename[:-3])
+          fullpath = os.path.join(dirname, filename)
+          relpath = fullpath[len(self.root) + 1 : -3]
+          testname = relpath.replace(os.path.sep, "/")
           test = testcase.TestCase(self, testname)
           tests.append(test)
     return tests