tests: make test_shellcheck depend on shellcheck

tests: make test_shellcheck depend on shellcheck

Since this test is only useful for development, don't fail the build on
platforms that lack shellcheck.

Another approach could be to add a dedicated make target to run the meta
tests but this was easier as a quick fix.

Change-Id: If9d30c7c4e3ee526f028ad82545e631ace272354
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>