Problem was block_size on partitioned tables was not set,
resulting in keys_per_block was not correct which affects
the cost calculation for read time of indexes (including
cost for group min/max).Which resulted in a bad optimizer
decision.

Fixed by setting stats.block_size correctly.

mysql-test/r/partition_range.result:
  Bug#48229: group by performance issue of partitioned table
  
  Added result
mysql-test/t/partition_range.test:
  Bug#48229: group by performance issue of partitioned table
  
  Added test
sql/ha_partition.cc:
  Bug#48229: group by performance issue of partitioned table
  
  Added missing assignment of stats.block_size.