Replace deprecated variable debug to debug_dbug on Galera tests

Clauses like SET GLOBAL debug = ''; should be replaced with SET_GLOBAL debug_dbug=''; as debug variable is deprecated and using it causes unnecessary warnings to result files.

(1) Grep all galera tests that use debug variable and replace it with debug_dbug
(2) Re-record changed test results (./mtr test --record)
(3) Run whole test suite to avoid regressions
(4) Push all this to staging branch e.g. bb-10.1-MDEV-18265 and set this MDEV to request review from me.
(5) After review fix review comments and push to main tree.