SimpleHTTPChannel for HTTP test helpers should be generalised
Currently SimpleHTTPChannel in http_helpers.py implements the http-return-json-headers test helper directly. As this test helper runs on port 80, the SimpleHTTP channel protocol should be generalised to allow for using the request line as a selector for other tests.
No existing implemented tests in ooni-probe request a resource other than the root resource ("/") and so the use of a selector will not mean changes are required to the implementation of those tests and this is also compatible with the specification of the http-return-json-headers helper.