REGTESTS: http-rules: test the declared capture slots

REGTESTS: http-rules: test the declared capture slots

The "declare capture" directive, the "id" form of the capture action
and the capture-req()/capture-res() converters had no coverage; only
"capture request header" and capture.req.hdr() were used elsewhere.

The test declares three request slots and two response slots in the
frontend, fills them from the frontend and from the backend, with the
action and with the converter, and reads them back with
capture.req.hdr() and capture.res.hdr(). It also checks that the
converters return their input unchanged, that an undeclared slot yields
nothing and that a slot whose input is missing stays empty.