Move the tests for split /\s/ and Unicode from split.t to split_unicode.t

Skip split_unicode.t under minitest, as it uses charnames, which uses
File::Spec, which may not be available.

[Experimentation reveals that git blame by default won't attribute lines past
this commit, unless --find-copies-harder is used. The alternative approach,
copy t/op/split.t as t/op/split_unicode.t as 1 commit, then prune both, fares
no better - by default git blame can't attribute through that *either*.
Again, --find-copies-harder works. Hence, do the partition as this one commit,
as it is simpler, and all other things are equal]

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