Add a test covering the highlighting of non-contiguous spans

[stof]

I ported this package to PHP as part of the scssphp project (as I'm porting dart-sass which uses this package). When collecting code coverage for the testsuite of my PHP port, I noticed that the code dealing with non-contiguous lines was not covered by the tests.
This PR adds a test covering this case.

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[stof]

Is anything missing here ? Can I get some feedback ?