Use MemoryError instead of unittest fail

This test does not inherit from a unittest, so it does not have a fail
method. This change makes the test raise a MemoryError with the existing
failure message.

Author: kdaily

tests/integration/customizations/s3/test_plugin.py

@@ -1554,7 +1554,7 @@ def assert_max_memory_used(self, process, max_mem_allowed, full_command):
                     peak_memory / 1024.0 / 1024.0,
                 )
             )
-            self.fail(failure_message)
+            raise MemoryError(failure_message)
 
     def test_transfer_single_large_file(self, files, shared_bucket):
         # 40MB will force a multipart upload.