Disable failing tests on Windows

[emillon]

This ensures that dune runtest works on Windows: if Windows runners are added to ocaml-ci, this will not break the build, and failures can be fixed incrementally.

[emillon]

Remark about the PR itself: I included 3 solutions depending on how you prefer:

1. individual (subdir) stanzas in the test dune file - this can be useful if you want to generate these
2. nested (subdir) stanzas in the test dune file - a bit more DRY
3. tests disabled in their own file close to the test. I think it's better since filenames are less duplicated (files can be moved more easily) but that's more files

If you prefer 1 we could extend the dune support for (applies_to) so that (cram (applies_to a/b)) is a shorthand for (subdirs a (cram (applies_to b))).

[voodoos]

I think I am more in favour of option 3 anyway.