Running language specs on Windows fails due to path separator differences

[stof]

Specs involving traces (in warnings or errors) and subfolders of the spec directory (in specs of @import in directives/import/error/conflict/index and directives/import/error/not_found/parent_relative, and maybe some equivalent test for @use if they exist) are failing when running them on Windows because the relative path shown in the stack trace involves a path separator, which is \ instead of / on Windows.

Maybe the runner could normalize path separators in the warning/error output before comparing it to the expected state. But it might cause other issues (or hide them if outputting broken code using / in a place where a \ was expected) if we have \ for other reason than a path separator, so it might require a more advanced normalization logic.