[red-knot] More comprehensive `is_assignable_to` tests (#15353)
## Summary
This changeset migrates all existing `is_assignable_to` tests to a
Markdown-based test. It also increases our test coverage in a hopefully
meaningful way (not claiming to be complete in any sense). But at least
I found and fixed one bug while doing so.
## Test Plan
Ran property tests to make sure the new test succeeds after fixing it.
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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
