REGTESTS: http: add a test to validate chunked responses delivery

REGTESTS: http: add a test to validate chunked responses delivery

I've had this test here never committed over the last 2.5 years, that
works fine and I didn't notice it was not part of the tree. It makes a
server return odd-sized chunked responses with short pauses between half
of thems and verifies they're not truncated on the client. It may detect
eventually state machine breakages, so better commit it.