#2966 was the `@[extern]` bug that prompted development of the
`test_extern` command, but then we merged the fix to #2966 without
updating the tests to use `test_extern`.
Fixes reference implementation of `ByteArray.copySlice`, as reported
https://github.com/leanprover/lean4/issues/2966.
Adds tests.
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Co-authored-by: Joachim Breitner <mail@joachim-breitner.de>