WIP: Fix missing borders in tables with border-collapse
This fixes a pair of bugs that caused some borders to be ignored by the border-collapse code. r? @notriddle
I haven't done a full test run with this change yet. After a `try` build I will update test expectations and/or add a new test.
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix#14834 (github issue number if applicable).
- [x] There are tests for these changes OR
- [ ] These changes do not require tests because _____
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