Extend loose index scan to support descending indexes.
This is achieved by removing a block skipping creating loose index
scan plan for descending index, as well as generalising the execution
of such plans.
The generalisation applies to all levels looking for min/max in loose
index scan. In the highest level (get_next), generalise min and max to
first and last, so that it still proceeds in the direction agreeing
with the index parity. In the lower levels, combine next_min and
next_max methods into next_min_max, and combine next_min_in_range and
next_max_in_range into next_min_max_in_range. This retains existing
logic of these functions and reduces code duplication, while allowing
handling of all four combinations (min, max) x (asc index, desc
index).
MyiSAM and Aria, indexes with prefix compression, where the first keypart
could be NULL - in this case they didn't expect the next key after the
not NULL key to be NULL.
Expect the first keypart of the next key to have zero length even
if store_not_null==1, this combination means keypart is NULL,
don't pack it.
also fixes MDEV-27340
Moved test from main suite to the new suites.
Move tests from maria/t and maria/r to maria
mysql-test/mysql-test-run.pl:
Added support for the new suites
