Create a compliance test suite

We need a test suite that concrete client implementors would be encouraged to run as part of their test suite to ensure the basic functionality of the HTTP client behaves correctly.

This coincides with the base functionality that libraries can rely on.

In practice, library authors, when receiving an issue, would first ensure that the concrete client their adopter is using complies with the test suite, and only then spend time debugging the issue. As libraries cannot work reliably with arbitrarily non-compliant client implementations, and we want to avoid them reverting to depending on a concrete client to achieve desired reliability.