Changelog: mix test reports doctests separately now (#7176)

Couldn't find it in the Changelog, maybe I'm a bit daft :) But the feature is there and I love it. Thanks 💚

```
tobi@comfy ~/github/benchee $ mix test
.............................................................................................................................................................................................................................................................................................

Finished in 2.3 seconds
102 doctests, 183 tests, 0 failures
```

Author: PragTob

CHANGELOG.md

@@ -182,6 +182,7 @@ Other improvements in Mix include better compiler diagnostics for editor integra
   * [mix profile.eprof] Add a new task for time-based profiling with eprof
   * [mix test] Run all functions in a describe block by giving the `file:line` the describe block starts
   * [mix test] Report the top N slowest tests with the `--slowest N` flag
+  * [mix test] Report the number of doctests and tests separately
   * [mix xref] Support `--include-siblings` in reports for umbrella support
   * [mix xref] Add `mix xref graph --format stats`
   * [mix xref] Add `--only-nodes` and `--label` filters to mix xref graph