Post-pushbuild fix

Added a purecov comment and a test for coverage of parallel
enable keys.


myisam/mi_check.c:
  Bug#4692 - DISABLE/ENABLE KEYS waste a space
  Added purecov comment.
mysql-test/r/myisam.result:
  Bug#4692 - DISABLE/ENABLE KEYS waste a space
  Added test result.
mysql-test/t/myisam.test:
  Bug#4692 - DISABLE/ENABLE KEYS waste a space
  Added test for coverage of parallel enable keys.