This happens when the `LessSourceMaps` global WE option is set.
When I refactor the settings logic, I should allow separate hard-coded logic for unit tests.

Regardless of the setting, that path is useless in real tests too.
We should change the compiler to pass `--source-map-basepath` so that it will write a relative path (we should pass the project root and allow the tests to override it).
The output will then be predictable for test cases.  (and we should in fact test that it's always a correct relative path)