Implement HTTP Test helper
Part of this is implemented here: https://gitweb.torproject.org/ooni-probe.git/blob/HEAD:/oonib/testhelpers/httph.py and some of this is specified here: https://trac.torproject.org/projects/tor/wiki/doc/OONI/Backend/HTTPServer
In particular it is required for header manipulation (#6077 (closed)) detection to have a HTTP test helper that will log the headers that are sent from the client and include them in the report.
Optionally if may be of use to have a way for the client to understand what is the content of the headers the backend has received (this is linked to #7231 (closed))
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