exec.sh: Support rust-coreutils as a multi-call binary
Since AT8605 (Reduce storage footprint of rust-coreutils), rust-coreutils installs 1 executable (/usr/bin/coreutils) and each utility is a link to that executable.
The exec.sh script needs access to /usr/bin/true which is a link to /usr/bin/coreutils and AppArmor needs the permissions to be applied to the realpath of the program being run, in this case: /usr/bin/coreutils.
The test now runs realpath on the $file value to support both situations.
This is a backport of !6 (merged) (merged)