Add test cases for bstRight, bstLeft

[MatthijsBlom]

Closes #1142

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[MatthijsBlom]

I'm not sure what to do with tests.toml.

[MatthijsBlom]

I have read the Exercism docs and the README of the problem-specifications repo, and it seems to me that tests.toml should be left as-is, as it has to do with problem specs synchronisation only.

I guessed adding myself to the contributors would be appropriate. Feel free to undo.