Updated tests for exercises using 'slice.equal()' [no important files changed]

[rmonnet]

After PR #161, I went through all the published exercises and checked if any other was using the 'slice.equal()' to compare expected vs result. I found another two and confirmed that the failed test report were confusing ('expected true but got
false').

[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!

[rmonnet]

I believe these were the last two remaining.

[glennj]

I also see flatten-array and grade-school:

$ cd exercises/practice/
$ grep -rl 'testing.expect.*slice.equal'
grade-school/grade_school_test.odin
flatten-array/flatten_array_test.odin
high-scores/high_scores_test.odin
anagram/anagram_test.odin

[rmonnet]

I did see these two but if you run odin test . on the solution stubs, it produces correct error messages so I didn't want to mess with them just for the sake of it.

Examples of output:

• grade_school.test_student_not_added_to_multiple_grades_in_the_roster
  expected [Grade{id = 2, students = ["Blair", "James"]}, Grade{id = 3, students = ["Paul"]}] got []

• flatten_array.test_5_level_nesting
  expected [0, 2, 2, 3, 8, 100, 4, 50, -2] got []