Add tests for `SassColor.interpolate()` without options #1604

Job	Run time
Unit tests	38s
Static analysis	24s
Lint spec files	18s
Language \| Dart Sass \| Dart ${{ matrix.dart_channel }}	0s
JS API \| Pure JS \| Node ${{ matrix.node_version }} \| ${{ matrix.os }}	0s
JS API \| Embedded \| Node lts/* \| ubuntu-latest	4m 53s
JS API \| Embedded \| Node lts/* \| windows-latest	7m 11s
JS API \| Pure JS \| Browser	0s
JS API \| Embedded \| Node lts/* \| macos-latest	3m 41s
JS API \| Embedded \| Node lts/-1 \| ubuntu-latest	5m 8s
JS API \| Embedded \| Node lts/-2 \| ubuntu-latest	5m 21s
JS API \| Embedded \| Node lts/-3 \| ubuntu-latest	1m 37s
	29m 11s

Provide feedback