Improve the test that was imported and adapted for MariaDB in
commit fb217449.

row_undo_step(): Move the DEBUG_SYNC point from trx_rollback_for_mysql().
This DEBUG_SYNC point is executed after rolling back one row.

trx_rollback_for_mysql(): Clarify the comments that describe the scenario,
and remove the DEBUG_SYNC point.

If the statement "if (trx->has_logged_persistent())" and its body are
removed from trx_rollback_for_mysql(), then the test
innodb.xa_recovery_debug will fail because the transaction would still
exist in the XA PREPARE state. If we allow the XA COMMIT statement
to succeed in the test, we would observe an incorrect state of the
XA transaction where the table would contain row (1,NULL). Depending
on whether the XA transaction was committed, the table should either
be empty or contain the record (1,1). The intermediate state of
(1,NULL) should never be observed after completed recovery.