filter.py: Fail if declared domains do not exist in the parent element

This patch also uncovered the fact that our test_filter_deps_ok() test
has been inaccurate. Thus the element built in this test
(deps-permitted.bst) has been modified so that it build depends on the
input.bst element, as it should.

tests/filter.py: Ensure deps_ok test passes
5 files changed