libtest-core: Mention bubblewrap as a user of this file

Signed-off-by: Simon McVittie <smcv@collabora.com>

tests/kolainst/libtest-core.sh

@@ -5,6 +5,7 @@
 #
 # Known copies are in the following repos:
 #
+# - https://github.com/containers/bubblewrap
 # - https://github.com/coreos/rpm-ostree
 #
 # Copyright (C) 2017 Colin Walters <walters@verbum.org>