Added HA_NULL_IN_KEY to table flags to allow for nullable unique indexes
and added test to verify

ha_federated.h:
  BUG #15133 "unique index with nullable value not accepted in federated table"
  added HA_NULL_IN_KEY to table flags to allow for nullable unique indexes
federated.test:
  BUG #15133 "unique index with nullable value not accepted in federated table"
  New test to show that nullable unique indexes work
federated.result:
  BUG #15133 "unique index with nullable value not accepted in federated table"
  New results for new test