zdtm: make grep_errors also grep warnings

It is inspired by the discussion about inotify fix:
https://github.com/checkpoint-restore/criu/pull/728#issuecomment-506929427

From one point of view, warnings might be important to understand why we
detect some visible change in the environment after c/r-ing the process,
and if this change is expected or not. So we should add "Warn" messages
to the output.

From over point, these warnings if they are expected, can spoil our
final logs with a lot of unnecessary details, so add changes in previous
patches to silence the most noisy of these warnings.

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

test/zdtm.py

@@ -2062,7 +2062,7 @@ def grep_errors(fname):
             before.append(l)
             if len(before) > 5:
                 before.pop(0)
-            if "Error" in l:
+            if "Error" in l or "Warn" in l:
                 if first:
                     print_fname(fname, 'log')
                     print_sep("grep Error", "-", 60)