Add a vsl_catchup varnishtest function
Add a vsl_catchup varnishtest function

This enables waiting for the logging thread to idle to make sure all
generated log is caught.

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