Document that we always use `do...end` for `before` in RSpec

[ci skip]
Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/development/testing.md

@@ -63,6 +63,8 @@ the command line via `bundle exec teaspoon`, or via a web browser at
 - Use `.method` to describe class methods and `#method` to describe instance
   methods.
 - Use `context` to test branching logic.
+- Use multi-line `do...end` blocks for `before` and `after`, even when it would
+  fit on a single line.
 - Don't `describe` symbols (see [Gotchas](gotchas.md#dont-describe-symbols)).
 - Don't assert against the absolute value of a sequence-generated attribute (see [Gotchas](gotchas.md#dont-assert-against-the-absolute-value-of-a-sequence-generated-attribute)).
 - Don't supply the `:each` argument to hooks since it's the default.