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

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