fix standard location of test file

[senekor]

The test files were renamed to use snake case in:
exercism/rust#2036

[github-actions]

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[senekor]

Bug report on the forum:
https://forum.exercism.org/t/i-dont-know-what-is-wrong-but-tests-failed-in-bottle-song/15976

[senekor]

This is not caught by the tests, because the tests actually read the slug from the test file name. And the existing two_plus_two actually was already snake case.

I don't think it's worth it to try to change the tests so they can detect future regressions like this, I see no reason the test file names would be changed again.