Add imenu support for deftest

[rrudakov]

A very small PR to make imenu work for deftest.

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• [x] The commits are consistent with our contribution guidelines.
• [x] You've run M-x checkdoc and fixed any warnings in the code you've written.
• [x] You've updated the changelog (if adding/changing user-visible functionality).
• [ ] You've updated the readme (if adding/changing user-visible functionality).

Thanks!