REGTESTS: silence warnings about content-type being ignored

REGTESTS: silence warnings about content-type being ignored

The following rules are triggering warnings about content-type being
ignored:

  http-request return content-type "text/plain" if { path /def-4 }
  http-request return content-type "text/plain" file /dev/null hdr "x-custom-hdr" "%[url]"  if { path /empty-file }

Annoyingly, the content-type is mandatory when the file is not empty,
that might be something to revisit in the future to relax at least one
of the rules so that the config doesn't strictly require to know the
file contents upfront.