defer delete converted strings in the tests in Raindrops

[0riginaln0]

https://exercism.org/tracks/odin/exercises/raindrops

[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.

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[0riginaln0]

closes #186

[rmonnet]

I concur with the change, the caller should be responsible for cleaning up the allocated string. But additional changes are required.

1. The .meta/example.odin file needs to be modified so it doesn't delete the strings.Builder variable containing the returned string.
2. An additional docs/instructions.append.md file must be created to explain that the responsibility to clean up the generated string memory belongs to the caller. (a couple of lines is all that is needed, look at the forth exercise for an example of format for this file).

Please make the requested changes and resubmit.

[rmonnet]

Latest commit looks good, merging now.