tests: fix isinstance test of wrong variable

3086a8627b2970cd7fbf49bc69413c08c68d5514 changed self._case to be a list, but
it was forgotten to adjust this line.

tests/run-tests.py

@@ -1770,7 +1770,7 class TTest(Test):
 
         if self._case:
             casestr = b'#'.join(self._case)
-            if isinstance(self._case, str):
+            if isinstance(casestr, str):
                 quoted = shellquote(casestr)
             else:
                 quoted = shellquote(casestr.decode('utf8')).encode('utf8')