REGTESTS: Make the script testing conditional set-var compatible with Vtest2

REGTESTS: Make the script testing conditional set-var compatible with Vtest2

VTest2 will replaced VTest in few months. There is not so much change
expected. One of them is that a User-Agent header is added by default in all
requests, except if an custom one is already set or if "-nouseragent" option
is used. To still be compatible with VTest, it is not possible to use the
option to avoid the header addition. So, a custom user-agent is added in the
last test of "sample_fetches/cond_set_var.vtc" to be sure it will pass with
Vtest and Vtest2. It is mandatory because the request length is tested.