Update CHANGELOG

Author: José Valim

CHANGELOG.md

@@ -40,6 +40,8 @@
   * [String] Deprecate `:global` option in `String.split/3` in favor of `parts: :infinity`
 
 * Backwards incompatible changes
+  * [ExUnit] `ExUnit.Test` and `ExUnit.TestCase` has been converted to structs
+  * [ExUnit] The test and callback context has been converted to maps
   * [Kernel] `File.Stat`, `HashDict`, `HashSet`, `Inspect.Opts`, `Macro.Env`, `Range`, `Regex` and `Version.Requirement` have been converted to structs. This means `is_record/2` checks will no longer work, instead, you can pattern match on them using `%Range{}` and similar
   * [URI] The `URI.Info` record has now become the `URI` struct
   * [Version] The `Version.Schema` record has now become the `Version` struct