Add tests for CSS `round()` with one argument #1650

Job	Run time
JS API \| Pure JS \| Node ${{ matrix.node_version }} \| ${{ matrix.os }}	0s
JS API \| Pure JS \| Browser	0s
JS API \| Embedded \| Node lts/* \| ubuntu-latest	4m 50s
Language \| Dart Sass \| Dart ${{ matrix.dart_channel }}	0s
JS API \| Embedded \| Node lts/* \| windows-latest	6m 31s
Lint spec files	18s
JS API \| Embedded \| Node lts/* \| macos-latest	4m 22s
Static analysis	20s
JS API \| Embedded \| Node lts/-1 \| ubuntu-latest	4m 46s
Unit tests	40s
JS API \| Embedded \| Node lts/-2 \| ubuntu-latest	4m 54s
JS API \| Embedded \| Node lts/-3 \| ubuntu-latest	5m 23s
	32m 4s

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