Consistent with MDEV-4206 and empty log_slow_filter still means
no explict filtering. Since 21518ab2 however the
log_queries_not_using_indexes became stored in the same variable.

As we need to test for the absense of log_queries_not_using_indexes
the SERVER_QUERY_NO_INDEX USED part of log_slow_statement, the empty
criteria resulted in an always true to log queries not using indexes if
log_slow_filter was set to empty.

Adjusted the log_slow.test for MDEV-4206 as slow_log_query has been
global and session for a while and it was relying on the MDEV-21187
buggy behavior to detect a slow query.

Reviewer: Monty