MDLSITE-4800 we already have clear rules for namespaces in **/tests directories. We already had (pretty much the same) rules for **/classes directories.
This is issue is about to make local_ci aware of them and verify they are correct for every file ever sent to any of those 2 directories.
Note, local_ci or whatever is decided to be better to control that aspect (say codechecker, say CiBoT...)
Sounds simple, sure it is not