zdtm: Distinguish between fail and crash of dump

Adds a exit_signal static method to criu_cli, criu_config and criu_rpc
used to detect a crash.

Fixes: #350

Signed-off-by: Bhavik Sachdev <b.sachdev1904@gmail.com>

test/zdtm/criu_config.py

@@ -41,3 +41,7 @@ class criu_config:
         if nowait:
             return cr
         return cr.wait()
+
+    @staticmethod
+    def exit_signal(ret):
+        return ret < 0