The original test case was lost when
merging WL#4571.

Added the testcase.


mysql-test/include/check_key_reads.inc:
  Bug#49369: No testcase for key caches on partitions
  
  New include file for testing key caches on partitions
  WL#4571
mysql-test/include/check_key_req.inc:
  Bug#49369: No testcase for key caches on partitions
  
  New include file for testing key caches on partitions
  WL#4571
mysql-test/r/partition_key_cache.result:
  Bug#49369: No testcase for key caches on partitions
  
  New test case for testing key caches on partitions
  WL#4571
mysql-test/t/partition_key_cache.test:
  Bug#49369: No testcase for key caches on partitions
  
  New test case for testing key caches on partitions
  WL#4571