REGTESTS: update http_reuse_be_transparent with "transparent" deprecated

REGTESTS: update http_reuse_be_transparent with "transparent" deprecated

With commit e93f3ea3f8 ("MEDIUM: proxy: deprecate the "transparent" and
"option transparent" directives") this one no longer works as the config
either has to be adjusted to use server 0.0.0.0 or to enable the deprecated
feature. The test used to validate a technical limitation ("transparent"
not supporting shared connections), indicated as being comparable to
"http-reuse never". Let's now duplicate the test for "http-reuse never"
and "http-reuse always" and validate both behaviors.

Take this opportunity to fix a few problems in this config:
  - use "nbthread 1": depending on the thread where the connection
    arrives, the connection may or may not be reused
  - add explicit URLs to the clients so that they can be recognized
    in the logs
  - add comments to make it clearer what to expect for each test