Add tests for underscores where hyphens aren't allowed #1508

Job	Run time
JS API \| Pure JS \| Browser	1m 27s
JS API \| Embedded \| Node lts/* \| ubuntu-latest	4m 49s
Language \| Dart Sass \| Dart stable	1m 59s
JS API \| Embedded \| Node lts/* \| windows-latest	6m 47s
JS API \| Pure JS \| Node lts/* \| ubuntu-latest	1m 34s
Lint spec files	15s
JS API \| Embedded \| Node lts/* \| macos-latest	3m 39s
Language \| Dart Sass \| Dart dev	1m 42s
JS API \| Pure JS \| Node lts/* \| windows-latest	4m 35s
JS API \| Embedded \| Node lts/-1 \| ubuntu-latest	4m 48s
JS API \| Pure JS \| Node lts/* \| macos-latest	1m 12s
JS API \| Embedded \| Node lts/-2 \| ubuntu-latest	5m 2s
JS API \| Pure JS \| Node lts/-1 \| ubuntu-latest	1m 34s
Static analysis	18s
Unit tests	42s
JS API \| Pure JS \| Node lts/-2 \| ubuntu-latest	1m 19s
JS API \| Embedded \| Node lts/-3 \| ubuntu-latest	5m 7s
JS API \| Pure JS \| Node lts/-3 \| ubuntu-latest	1m 51s
	48m 40s

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