Fix -moz-outline-radius shorthand serialization
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These changes aim to solve #15169 correcting the `ToCss` implementation for `LonghandsToSerialize` for the `border-radius` shorthands. They also reduce redundant values like `1px 2px 1px 2px` to `1px 2px` to either sides of the slash.
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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#15169 (github issue number if applicable).
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- [x] There are tests for these changes
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