D31084 put `atf-pytest-wrapper` in the `/usr/tests`. It is causing problems with `distribution` target.
While this can be fixed by altering `DISTRIBUTION`, `${PACKAGE}FILESDIR` and a couple of other variables in the Makefile, it doesn't look like a proper way of doing things.
Target Makefile would need to replicate some logics from `bsd.test.mk` which is not desired.
Instead, switch to a cleaner approach by moving this wrapper to the `atf-sh` wrapper in `libexec/atf`.
This way it's logically tied to our in-base version of atf and constantly installed with other atf wrappers to `/usr/libexec`.