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