REGTESTS: http: Create a dedicated script to test spliced bodyless responses

REGTESTS: http: Create a dedicated script to test spliced bodyless responses

Splicing is not available on all platform. Thus a dedicated script is used
to check we properly skip payload for bodyless response when splicing is
used. This way, we are still able to test the feature with the original
script on all platform.

This patch fixes an issue on the CI introduced by commit ef2b15998
("BUG/MINOR: htx/mux-h1: Properly handle bodyless responses when splicing is
used"). It must be backported with the above commit.