bug(core): test_unicode needs ICU4C 76 with Unicode 16.0.0

From upgrading to Unicode 16.0 in #12393, the following core test failed

>
> ```
> Test failed at ../../../tests/unit/ldml/test_unicode.cpp:175:
> 02:45:08  expected: 16
> 02:45:08  actual: 15
> ```

Per @srl295
so… the test is working as designed.
The problem here is that it needs ICU4C v76 which isn't release for another month, and then Node.js based on that.

I assume keyman 18 release will be after those dates, so perhaps we need some way to skip this test until a later phase?

_Originally posted by @srl295 in https://github.com/keymanapp/keyman/issues/12393#issuecomment-2343697801_