Support setting configuration in update_shaping_tests input

[khaledhosny]

Support things like:

[configuration]
forbidden_glyphs = [
    ".notdef",
]

Does not use [configuration.defaults] when found, though, as it is redundant since direction etc. can be set in [input]

Checklist

• [x] update CHANGELOG.md
• [ ] wait for the tests to pass
• [ ] request a review