REGTEST: add a basic test for HTTP rules manipulating headers

REGTEST: add a basic test for HTTP rules manipulating headers

There is always a risk of breaking HTTP processing when performing certain
code changes. This test modifies a request's start line, uses variables,
adds and modifies headers, interleaves them with the start-line changes,
and makes use of different header formats involving duplicated names,
duplicated values, empty fields and spaces around values. These operations
are performed both in the frontend and in the backend, for both the request
and the response. A CRC is computed on the concatenation of all the values,
and the concatenations are sent as individual header fields as well to help
debugging when the test fails.

The test reliably works since 1.6, implying that the HTTP processing did
not change. It currently fails on HTX.