Clarify non-Unicode Korean tests

[staticfloat]

This is the recommended locale by Arch Linux, so let's look for it as
well

[staticfloat]

This, combined with a rootfs image bump including the bits from JuliaCI/rootfs-images#208 will help us to silence CI warnings such as https://buildkite.com/julialang/julia-master/builds/13227#01819c05-81fc-4bd9-bb58-7f7872193d6c/560-843

[vtjnash]

This would sort of defeat the comment there which says not to use Unicode here

[staticfloat]

I was confused by that comment, because the output of our tests seems to be identical across every korean locale I can find:

julia> korloc = ["ko_KR.UTF-8", "ko_KR.EUC-KR", "ko_KR.CP949", "ko_KR.949", "Korean_Korea.949"]
       withlocales(korloc) do locale
           s = Libc.strftime(0.0)
           @info(locale, s, bytes2hex(sha256(s)))
       end
┌ Info: ko_KR.UTF-8
│   s = "1970년 01월 01일 (목) 오전 12시 00분 00초"
└   bytes2hex(sha256(s)) = "d0117d51da3a9a0953e0b504207190950c3d664688743a103c95584148f99092"
┌ Info: ko_KR.EUC-KR
│   s = "1970년 01월 01일 (목) 오전 12시 00분 00초"
└   bytes2hex(sha256(s)) = "d0117d51da3a9a0953e0b504207190950c3d664688743a103c95584148f99092"
┌ Info: ko_KR.cp949
│   s = "1970년 01월 01일 (목) 오전 12시 00분 00초"
└   bytes2hex(sha256(s)) = "d0117d51da3a9a0953e0b504207190950c3d664688743a103c95584148f99092"

This was generated with a locale.gen that I made that looks like the following:

ko_KR.EUC-KR EUC-KR
ko_KR.UTF-8 UTF-8
ko_KR.cp949 CP949

If we're wanting to test codepoint output that is explicitly not UTF-8, I think we need a different test, because these all output the exact same UTF-8.

[vtjnash]

yes, exact equivalence would be rather the point of the test (#27273)

[staticfloat]

Alright, I've switched this PR to instead clarify some things and adds tests (where appropriate) to ensure that we don't just get en_US.UTF-8 back. I've also updated the Linux testers to use the new rootfs images that include ko_KR.EUC-KR baked in. We should see the linux testers now no longer print the warning that these tests are being skipped.

[staticfloat]

Confirmed working. Compare the previous master build:

https://buildkite.com/julialang/julia-master/builds/13308#0181ad71-e4f7-4efe-be37-0d90aa7357c3/568-842

Versus this build:

https://buildkite.com/julialang/julia-master/builds/13325#0181b08b-f047-42c0-a3c3-500d6f325827/564-838

Windows builds also seem good.

[staticfloat]

@vtjnash Does this look better now? Thanks for helping me to understand the purpose behind this test. I've done my best to encode that in comments in the source as well.