REGTESTS: lua: fix tune.lua.openlibs in Lua reg-tests

These tests were using "tune.lua.openlibs none" with lua-load, which
was a no-op in the old code since Lua states 0 and 1 were always
initialised before config parsing with all standard libraries.

Now that the Lua VM is initialised lazily, the restriction correctly
applies to state 0 as well. Replace "none" with the minimal set of
libraries actually required by each test's Lua code:

  - lua_socket.vtc, h_txn_get_priv.vtc, lua_httpclient.vtc: string
  - txn_get_priv.vtc: string,table

reg-tests/lua/h_txn_get_priv.vtc

@@ -9,7 +9,7 @@ haproxy h1 -conf {
         thread-groups 1
     .endif
 
-        tune.lua.openlibs none
+        tune.lua.openlibs string
         tune.lua.bool-sample-conversion normal
         lua-load ${testdir}/h_txn_get_priv.lua
 

reg-tests/lua/lua_httpclient.vtc

@@ -42,7 +42,7 @@ haproxy h1 -conf {
         thread-groups 1
     .endif
 
-        tune.lua.openlibs none
+        tune.lua.openlibs string
         tune.lua.bool-sample-conversion normal
         lua-load ${testdir}/lua_httpclient.lua
 

reg-tests/lua/lua_socket.vtc

@@ -14,7 +14,7 @@ haproxy h1 -conf {
         thread-groups 1
     .endif
 
-        tune.lua.openlibs none
+        tune.lua.openlibs string
         tune.lua.bool-sample-conversion normal
         lua-load ${testdir}/lua_socket.lua
 

reg-tests/lua/txn_get_priv.vtc

@@ -10,7 +10,7 @@ haproxy h1 -conf {
         thread-groups 1
     .endif
 
-        tune.lua.openlibs none
+        tune.lua.openlibs string,table
         tune.lua.bool-sample-conversion normal
         lua-load ${testdir}/txn_get_priv.lua
         lua-load ${testdir}/txn_get_priv-print_r.lua