Use python httpbin test suite

[gaul]

It would be nice to use the official httpbin test suite instead of inventing new test cases. This might require a forked submodule like S3Proxy does with s3-tests.

[gaul]

https://github.com/gaul/go-httpbin/tree/java-httpbin has workarounds to test java-httpbin.

[gaul]

https://github.com/gaul/httpbin/tree/java-httpbin also has workarounds to test java-httpbin.

[gaul]

This is more attractive now that httpbin maintenance is active at https://github.com/psf/httpbin.