MDEV-33281 Make BNL() hint work for join_cache_level=0

join_cache_level=0 disables join cache buffers, but the hint
BNL() now allows to employ BNL(H) buffers for particular tables
or query blocks.

This commit also adds a number of test cases including
OUTER JOINs to make sure hints do not break the rules of
join buffers application

Author: Olernov