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:

• AsyncPayloadProcessor owned payloads are released too early #111
• Added reader rewind to remote payload processor #112
• Revert PayloadProcessor ModelId to remain consistent with original value #113
• fix: Fix PayloadProcessor response payload race condition #120
• feat: Add vModelId to PayloadProcessor Payload #123

Originally posted by @njhill in #112 (comment)