Clean up streamable session after test

[findleyr]

Describe the bug
The conformance test harness does not issue an HTTP DELETE cleaning up its session after it runs.

To Reproduce
Steps to reproduce the behavior:

1. Run the conformance test against a server.
2. Check to see if that server receives a DELETE.

Expected behavior
As much as possible, the conformance test should endeavor to be hermetic, and should clean up when it is done.

Additionally, there should probably be a conformance test for session lifecycle!

Logs
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Additional context
Add any other context about the problem here.