vtc: Stabilize r02310.vtc

The test is racy, we should always perform the backend fetch to reach
the "barrier b1 sync" in the server body, otherwise the test will be
stuck on the second barrier sync until timeout.

Fixes: #4098

Author: walid-git

bin/varnishtest/tests/r02310.vtc

@@ -11,6 +11,7 @@ server s1 {
 varnish v1 -vcl+backend {} -start
 
 varnish v1 -cliok "param.set feature +http2"
+varnish v1 -cliok "param.set feature -vcl_req_reset"
 
 client c1 {
 	send "GET / HTTP/1.1\r\n"