Deleting unnecessary to_string() calls in the test file

[yhuang912]

This allow the implementation to use &str in place of String. Making both the implementations and the test code cleaner, and serve a good exercise to work out the lifetimes.

The change in the test file will affect existing passing solutions since they had to make attrs field of type HashMap<String, String>.

[github-actions]

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[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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