tests/libtest-core.sh: Add a comment that this copy is canonical
tests/libtest-core.sh: Add a comment that this copy is canonical

Ref: https://github.com/projectatomic/bubblewrap/pull/203

Closes: #1047
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree