Now that I have removed the barrier to testing this in the buildfarm, and added an appropriate locale setting to drongo, we can see that this test fails like this:
Further investigation shows that if we change the two instances of "de_DE" to "de-DE" the tests behave as expected, so it appears that while POSIX style aliases have been created for the BCP 47 style locales, using the POSIX aliases doesn't in fact work. I cant see anything that turns the POSIX locale name back into BCP 47 at the point of use, which seems to be what's needed.
The command that's failing is "SET lc_time TO 'de_DE';", and that area of code is untouched by this patch. As mentioned in [1], the problem seems to come from a Windows bug that the CI images and my development machines have patched out.
I think we should change the locale name to make the test more robust, as the attached. But I don't see a problem with making an alias for the collations.