REGTESTS: launch http_reuse_always in mworker mode

We don't have enough tests with the mworker mode, and even less that
have no master CLI (-S) configured. Let's run this one with -W, it
shouldn't have any impact.

VTest won't be able to catch a lot of things for now, but that's a first
step.

reg-tests/connection/http_reuse_always.vtc

@@ -4,7 +4,7 @@ varnishtest "Test the http-reuse always mode"
 
 feature ignore_unknown_macro
 
-haproxy h1 -conf {
+haproxy h1 -W -conf {
 	defaults
 		mode http