REGTESTS: restrict execution to a single thread group

When threads are enabled and running on a machine with multiple CCX
or multiple nodes, thread groups are now enabled since 3.3-dev2, causing
load-balancing algorithms to randomly fail due to incoming connections
spreading over multiple groups and using different load balancing indexes.

Let's just force "thread-groups 1" into all configs when threads are
enabled to avoid this.

reg-tests/lua/lua_socket.vtc

@@ -10,6 +10,10 @@ server s1 {
 
 haproxy h1 -conf {
     global
+    .if feature(THREAD)
+        thread-groups 1
+    .endif
+
         tune.lua.bool-sample-conversion normal
         lua-load ${testdir}/lua_socket.lua