Bug#46754: 'rows' field doesn't reflect partition pruning

The EXPLAIN's result in 'rows' field
was evaluated to number of rows when the table was opened
(not from the table cache) and only the partitions left
after pruning was updated with its correct number
of rows.

The evaluation of the 'rows' field was using handler::records()
which is a potentially expensive call, and ignores the partitioning
pruning.

The fix was to use the handlers stats.records after updating it
with ::info(HA_STATUS_VARIABLE) instead.

mysql-test/r/partition_pruning.result:
  updated result
mysql-test/t/partition_pruning.test:
  Added test.
sql/sql_select.cc:
  Use ::info + stats.records instead of ::records().