martian: run unit tests againts http2 server

[mmatczuk]

Previously we added tests running over TLS and against HTTP handler implementation.
Is's important to also run the tests against add HTTP2 server.

This is bit complicated as Go does not support http2 proxy servers out of the box,
and adding it would require tests fork or refactoring as we cannot send plain text.
Maybe we could make use of a translation layer or come with another trick to minimize the amount of changes.