Make sure the tests displays unicode characters properly [no important files changed]

[rmonnet]

The generic 'expect_slices_match()' routine use the '%v' format, when applied to string with unicode, this displays the code points rather than the unicode characters.

For example, the test 'test_handles_case_of_greek_letters' display the expected result as
'["\u0392\u0393\u0391", "\u03b3\u03b2\u03b1"].
I modified the 'expect_slices_match()' procedure to use strings and '%s' instead and now the expected result looks like '[ΒΓΑ, γβα]'.

[no important files changed]

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

For more information, refer to the documentation. If you are unsure whether to add the message or not, please ping @exercism/maintainers-admin in a comment. Thank you!

[glennj]

2 things:

1. can you please make a note in dev/docs/faqs/when_writing_tests_how_to_compare_result_and_expected_for_slices.md -- it's good to keep a central library for these custom testing functions and the dev docs is as good a place as any.

2. the "no important files changed" message goes into the merge message. The merge message becones the commit message for main's HEAD, and whatever backend process Exercism uses looks there to decide whether to retest all submissions. Since the track isn't even launched, there's not much overhead to test them all.

[rmonnet]

Good point! I included a new template file with the expect_slices() and expect_string_slices() procedures for developers to copy directly and modified the When Testing How to Compare Result and Expected for Slices FAQ accordingly.