Replace newline chars with newlines in test report

When the library / app under test returns long error messages on test
failures it will often include newlines.  These are today displayed as
`\\n` and we get an error message on one very long line.

This commit turns these newline characters into actual newlines, which
makes the content of the `*cider-test-report*` buffer much more pleasant
to read.

Author: expez

CHANGELOG.md

@@ -3,6 +3,10 @@
 ## master (unreleased)
 - Allow using `npx nbb` as `cider-nbb-command`.
 
+### Changes
+
+- [#3281](https://github.com/clojure-emacs/cider/pull/3281): Replace newline chars with actual newlines in `*cider-test-report*` buffer, for prettier error messages.
+
 ## 1.6.0 (2022-12-21)
 
 ### New features

cider-test.el

@@ -478,6 +478,12 @@ With the actual value, the outermost '(not ...)' s-expression is removed."
                       (cider-test-render-assertion buffer test)))))
               vars))
            results)))
+      ;; Replace any newline chars with actual newlines to make long error
+      ;; messages more readable
+      (goto-char (point-min))
+      (while (search-forward "\\n" nil t)
+        (replace-match "
+"))
       (goto-char (point-min))
       (current-buffer))))