Include vcl.use in varnish -vcl failure detection
Include vcl.use in varnish -vcl failure detection

Instead of crashing the varnishtest sub-process with an assert,
check that both `vcl.load` and `vcl.use` pass or fail when running
the varnish command for `-vcl` and `-errvcl`. `vcl.use` may fail
the warmup step.

feld/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache,gquintard/Varnish-Cache,feld/Varnish-Cache,feld/Varnish-Cache,feld/Varnish-Cache,gquintard/Varnish-Cache