Test that sassTrue and sassFalse are instances of SassBoolean #1679

Job	Run time
Static analysis	18s
Unit tests	37s
Lint spec files	16s
Language \| Dart Sass \| Dart stable	59s
Language \| Dart Sass \| Dart dev	1m 3s
JS API \| Pure JS \| Node lts/* \| ubuntu-latest	1m 14s
JS API \| Pure JS \| Browser	1m 16s
JS API \| Embedded \| Node ${{ matrix.node_version }} \| ${{ matrix.os }}	0s
JS API \| Pure JS \| Node lts/* \| windows-latest	3m 29s
JS API \| Pure JS \| Node lts/* \| macos-latest	1m 31s
JS API \| Pure JS \| Node lts/-1 \| ubuntu-latest	1m 15s
JS API \| Pure JS \| Node lts/-2 \| ubuntu-latest	1m 16s
JS API \| Pure JS \| Node lts/-3 \| ubuntu-latest	1m 19s
	14m 33s

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