run-tests: handle json.dumps divergence

In py2, json.dumps includes a trailing space after a comma at the
end of lines. The py3 behavior which omits the trailing space is
preferable, so we're going to strip it.

tests/run-tests.py

@@ -1836,7 +1836,8 b' class TextTestRunner(unittest.TextTestRu'
                                 tres = {'result': res}
 
                             outcome[tc.name] = tres
-                    jsonout = json.dumps(outcome, sort_keys=True, indent=4)
+                    out = json.dumps(outcome, sort_keys=True, indent=4)
+                    jsonout = '\n'.join([l.rstrip() for l in out.splitlines()])
                     fp.writelines(("testreport =", jsonout))
 
             self._runner._checkhglib('Tested')