Fixes <https://bugs.gnu.org/32184>.
* guix/tests.scm (call-with-external-store): New procedure.
(with-external-store): New macro.
* tests/pack.scm (%store): Remove.
(test-assertm): Add 'store' parameter.
("self-contained-tarball"): Wrap in 'with-external-store'.
* tests/guix-pack.sh: Connect to the external store, if possible, by
setting NIX_STORE_DIR and GUIX_DAEMON_SOCKET. Remove most uses of
'--bootstrap'.