Add tests for a plain CSS `@import url("foo")` #1637

Job	Run time
Language \| Dart Sass \| Dart ${{ matrix.dart_channel }}	0s
JS API \| Embedded \| Node lts/* \| ubuntu-latest	4m 51s
JS API \| Pure JS \| Browser	0s
JS API \| Pure JS \| Node ${{ matrix.node_version }} \| ${{ matrix.os }}	0s
JS API \| Embedded \| Node lts/* \| windows-latest	7m 7s
JS API \| Embedded \| Node lts/* \| macos-latest	3m 42s
Lint spec files	17s
JS API \| Embedded \| Node lts/-1 \| ubuntu-latest	4m 53s
Static analysis	24s
Unit tests	36s
JS API \| Embedded \| Node lts/-2 \| ubuntu-latest	5m 8s
JS API \| Embedded \| Node lts/-3 \| ubuntu-latest	1m 36s
	28m 34s

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