Problem: we don't prune a LESS THAN partition if MAXVALUE is given and
given value is equal to a LESS THAN value.

Fix: prune partitions in such cases.


mysql-test/r/partition.result:
  Fix for bug#42944: partition not pruned correctly
    - test result.
mysql-test/t/partition.test:
  Fix for bug#42944: partition not pruned correctly
    - test case.
sql/sql_partition.cc:
  Fix for bug#42944: partition not pruned correctly
    - prune partition if given value is equal to a LESS THAN value
      and it's not a "PARTITION ... LESS THAN MAXVALUE" one.