luacheck: change global vars to local in sql-tap

Fixed luacheck warning 111 (setting non-standard global variable)
in test/sql-tap directory.
Enabled this directory for checking W111 in
config file(.luacheckrc).

Changed almost all variables in test/sql-tap from globals
to locals. In any cases, where variables need to be global,
added explicit _G. prefix (table of globals).

Fixes #5173
Part-of #5464