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

So some tests have been removed.
diff --git a/reg-tests/seamless-reload/b00000.vtc b/reg-tests/seamless-reload/b00000.vtc
index 8f7acf6..35ec0e9 100644
--- a/reg-tests/seamless-reload/b00000.vtc
+++ b/reg-tests/seamless-reload/b00000.vtc
@@ -22,6 +22,7 @@
 
   defaults
     mode http
+    ${no-htx} option http-use-htx
     log global
     option httplog
     timeout connect         15ms