test: Do not expect coverage of test failure code path.

caucase/test.py

@@ -288,7 +288,7 @@ def print_buffer_on_error(func):
     """
     try:
       return func(self, *args, **kw)
-    except Exception:
+    except Exception: # pragma : no cover
       sys.stdout.write(utils.toBytes(os.linesep))
       sys.stdout.write(self.caucase_test_output.getvalue())
       raise