Auto merge of #14035 - Permutatrix:iss-14030, r=emilio

Don't mark the first/last fragment of an {ib} split as FIRST/LAST_FRAGMENT_OF_ELEMENT.

<!-- Please describe your changes on the following line: -->
This change allows inline margins, borders, and padding to interact correctly with {ib} splits.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14030
- [X] There are tests for these changes

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tests/wpt/metadata-css/css21_dev/html4/block-in-inline-insert-017.htm.ini

@@ -1,3 +0,0 @@
-[block-in-inline-insert-017.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/inline-box-001.htm.ini

@@ -1,3 +0,0 @@
-[inline-box-001.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/ltr-ib.htm.ini

@@ -1,3 +0,0 @@
-[ltr-ib.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/rtl-ib.htm.ini

@@ -1,3 +0,0 @@
-[rtl-ib.htm]
-  type: reftest
-  expected: FAIL