REGTESTS: connection: disable http_reuse_be_transparent.vtc if !TPROXY

REGTESTS: connection: disable http_reuse_be_transparent.vtc if !TPROXY

http_reuse_be_transparent.vtc relies on "transparent" proxy option which
is guarded by the USE_TPROXY ifdef at multiple places in the code.

Hence, executing the above test when haproxy was compiled without the
USE_TPROXY feature (ie: generic target) results in this kind of error:

  ***  h1    debug|[NOTICE]   (1189756) : haproxy version is 2.9-dev1-8fc21e-807
  ***  h1    debug|[NOTICE]   (1189756) : path to executable is ./haproxy
  ***  h1    debug|[ALERT]    (1189756) : config : parsing [/tmp/vtc.1189751.18665e7b/h1/cfg:11]: option 'transparent' is not supported due to build options.
  ***  h1    debug|[ALERT]    (1189756) : config : Error(s) found in configuration file : /tmp/vtc.1189751.18665e7b/h1/cfg

Now we skip the regtest if TPROXY feature is missing.