REGTEST: Add an HTX reg-test to check an edge case

REGTEST: Add an HTX reg-test to check an edge case

This test checks that an HTTP message is properly processed when we failed to
add the HTX EOM block in an HTX message during the parsing because the buffer is
full. Some space must be released in the buffer to make it possible. This
requires an extra pass in the H1 multiplexer. Here, we must be sure the mux is
called while there is no more incoming data.

It is a "devel" test because conditions to run the test successfully is highly
dependent on the implementation. So if it fail, it is not necessarily a bug. It
may be due of an internal change. It relies on internal HTX sample fetches.