REGTEST: http-messaging: add "option http-buffer-request" for H2 tests

REGTEST: http-messaging: add "option http-buffer-request" for H2 tests

These tests upload contents and randomly make the server start to
respond before the client finishes to upload data, making the test
occasionally fail. Waiting for a body in the server doesn't always
work, depending on the method or how the data are advertised. Thus,
let's ask haproxy to wait for the request using the aforementioned
option, it guarantees that the DATA frame is sent before the response
HEADERS frame is delivered.