REGTEST: Add option to use HTX prefixed by the macro 'no-htx'

So some tests have been removed.
diff --git a/reg-tests/http-messaging/h00002.vtc b/reg-tests/http-messaging/h00002.vtc
index 6fc536a..27d2540 100644
--- a/reg-tests/http-messaging/h00002.vtc
+++ b/reg-tests/http-messaging/h00002.vtc
@@ -1,4 +1,4 @@
-varnishtest "HTTP request tests: H2 to H1, legacy mode"
+varnishtest "HTTP request tests: H2 to H1 (HTX and legacy mode)"
 #REQUIRE_VERSION=1.9
 
 # Run it with HAPROXY_PROGRAM=$PWD/haproxy varnishtest -l -k -t 1 "$1"
@@ -41,7 +41,7 @@
 	#log stdout format raw daemon
 	mode http
 	option http-buffer-request
-	#option http-use-htx
+	${no-htx} option http-use-htx
 	timeout connect 1s
 	timeout client  1s
 	timeout server  1s