REGTEST/MINOR: Set HAPROXY_PROGRAM default value.

With this patch, we set HAPROXY_PROGRAM environment variable
default value to the haproxy executable of the current working directory.
So, if the current directory is the haproxy sources directory,
the reg-tests Makefile target may be run with this shorter command:

  $ VARNISTEST_PROGRAM=<...> make reg-tests

in place of

  $ VARNISTEST_PROGRAM=<...> HAPROXY_PROGRAM=<...> make reg-tests
1 file changed