Always return test documentation messages as strings

[mallt]

• The commits are consistent with our contribution guidelines
• You've added tests to cover your change(s)
• All tests are passing
• The new code is not generating reflection warnings
• You've updated the README (if adding/changing middleware)

This PR fixes cider issue 2449 by making sure test documentation messages are always returned as strings.

[bbatsov]

Please, update the changelog as well.

[bbatsov]

I don't think the ->> reads very well here (because both functions have non-trivial bodies and you're also nesting threadings), so you might want to extract a couple of helper functions or something like this.

[codecov]

Codecov Report

Merging #550 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #550      +/-   ##
=========================================
+ Coverage   73.02%   73.1%   +0.07%     
=========================================
  Files          36      36              
  Lines        2369    2376       +7     
  Branches      137     137              
=========================================
+ Hits         1730    1737       +7     
  Misses        502     502              
  Partials      137     137

Impacted Files	Coverage Δ
src/cider/nrepl/middleware/test.clj	59.39% <100%> (+1.49%)	⬆️

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[bbatsov]

Thanks!