matching-brackets: Sync tests and add test template

[ellnix]

Some shenanigans was going on where tests were missing in tests.toml but were actually implemented in the test file. With this everything is consistent.

Specifically
fn early_incomplete_brackets() {}
fn early_mismatched_brackets() {}

[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]

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[ellnix]

I don't think any solutions that were working will now break, but as there is technically a new test I think the tests should run again.