If we really want to adhere to best working practices, then it should be a separate team performing the tests based on the same specifications that the develope

If we really want to adhere to best working practices, then it should be a separate team performing the tests based on the same specifications that the developers used to write the code (ie the testing team wouldn't perform tests based on the developers specifications but rather the project owners specs to mitigate the risk of the developers misunderstanding the specs). However that requires a level of process that is either too costly, or too rigid for many IT companies to follow.