caddy-frontend: Trim down SlapOS log on error

In case of error during processing SlapOS software or instance expose
last 100 lines of the message, which is enough to identify the problem and
does not clutter the test master.

software/caddy-frontend/test/utils.py

@@ -177,7 +177,8 @@ class SlapOSInstanceTestCase(unittest.TestCase):
         cls.config, environment=os.environ)
       stream.seek(0)
       stream.flush()
-      assert cls.software_status_dict['status_code'] == 0, stream.read()
+      message = ''.join(stream.readlines()[-100:])
+      assert cls.software_status_dict['status_code'] == 0, message
     finally:
       logger.removeHandler(stream_handler)
       del stream
@@ -198,7 +199,8 @@ class SlapOSInstanceTestCase(unittest.TestCase):
         environment=os.environ)
       stream.seek(0)
       stream.flush()
-      assert cls.instance_status_dict['status_code'] == 0, stream.read()
+      message = ''.join(stream.readlines()[-100:])
+      assert cls.instance_status_dict['status_code'] == 0, message
     finally:
       logger.removeHandler(stream_handler)
       del stream