diff --git a/tests/run-tests.py b/tests/run-tests.py
--- a/tests/run-tests.py
+++ b/tests/run-tests.py
@@ -2356,9 +2356,8 @@
         # assume all tests in same folder for now
         if testdescs:
             pathname = os.path.dirname(testdescs[0]['path'])
-            if pathname and not osenvironb[b'TESTDIR'].endswith(b'/'):
-                osenvironb[b'TESTDIR'] += b'/'
-            osenvironb[b'TESTDIR'] += pathname
+            osenvironb[b'TESTDIR'] = os.path.join(osenvironb[b'TESTDIR'],
+                                                  pathname)
         if self.options.outputdir:
             self._outputdir = canonpath(_bytespath(self.options.outputdir))
         else: