- Add new mysqltest command "mkdir" and "rmdir"


client/CMakeLists.txt:
  Build mysys/my_mkdir.c with mysqltest
client/Makefile.am:
  Build mysys/my_mkdir.c with mysqltest
client/mysqltest.c:
  Add new mysqltest commands "mkdir" and "rmdir"
mysql-test/t/mysqltest.test:
  Add tests for "mkdir" and ""rmdir"