Add test for Unicode named subpatterns and property mixes

[BenjaminGalliot]

This new test is related to issues #35322/#35459 (Unicode named subpatterns) and #40231 (property mixes of Unicode scripts and classes), and PR #39310.

[BenjaminGalliot]

Should this PR be added to the 1.6.1 backport (#40209)? I think it could be good to link them.

[KristofferC]

This PR doesn't work as is on 1.6 because it uses API that doesn't exist there. If you make a PR against that branch with the older API, it can be backported.

[BenjaminGalliot]

Yes, sorry, I forgot this last line in the test would be invalid for 1.6! I just made a small PR specifically for the backport branch.

[vtjnash]

I think in general, we don't need to backport test-only PRs, though no harm in merging yours!

[BenjaminGalliot]

That's what I thought too, but as I had seen in this current backport list a PR with a one-character spelling correction, I thought it might be worth it, especially as if I had done this test earlier, we would have noticed this breaking change which generated a not so obvious bug in one of my projects! :-)