Add unit test for Payload Processor

[ckadner]

It would also be useful to have a unit test for this, but the tests included here don't exercise the actual bug.

Ideally we'd have a test that actually runs a local modelmesh with an asyncprocessor configured and can trigger the problem (other unit tests already run modelmesh to test other stuff and could hopefully be used as a starting point and adapted).

Related issues:

• #111
• #112
• #113
• #120
• #123

Originally posted by @njhill in https://github.com/kserve/modelmesh/issues/112#issuecomment-1813531714