reftest: add a test showing root redirection on Windows when OPAMROOT contains spaces

[kit-ty-kate]

Split off from #5457 as we're not yet sure how to properly test this

[dra27]

Parking a reference to a commit which started updating this test further - https://github.com/dra27/opam/commit/f3c0b69abd2d53e8599f3c0e76548575d8aed696 (cf. https://github.com/dra27/opam/pull/22)