cmp_ds_cont has never worked
The "cmp_ds_cont" function in the zfs-test suite ends with a call to "echo". As a result, any consumers of this function interpreting its return code to mean "success" or "failure" would have been flawed (i.e. if it was used in conjunction with "log_must"), as the return code of the function would have been the return code of the "echo" command; i.e. it would almost always report success.