Add a note to testing docs about the `:each` argument to RSpec hooks

[ci skip]

doc/development/testing.md

@@ -64,6 +64,7 @@ the command line via `bundle exec teaspoon`, or via a web browser at
   methods.
 - Use `context` to test branching logic.
 - Don't `describe` symbols (see [Gotchas](gotchas.md#dont-describe-symbols)).
+- Don't supply the `:each` argument to hooks since it's the default.
 - Prefer `not_to` to `to_not`.
 - Try to match the ordering of tests to the ordering within the class.
 - Try to follow the [Four-Phase Test][four-phase-test] pattern, using newlines