run-tests: convert the remaining os.system() call to Unicode

I wasn't able to hit this path in 543a788eea2d, but I have now when I
accidentally left off `--local`.

tests/run-tests.py

@@ -2976,7 +2976,7 b' class TestRunner(object):'
         makedirs(self._bindir)
 
         vlog("# Running", cmd)
-        if os.system(cmd) == 0:
+        if os.system(_strpath(cmd)) == 0:
             if not self.options.verbose:
                 try:
                     os.remove(installerrs)