Refactor matcher_spec.

- Use a helper method to create the matchers, rather than
  `Matcher.new` + `for_expected`. I plan to rewrite the
  implementation of `Matcher` in a way that will require it
  to be instantiated differently and I having this helper
  method gives us only one place to update.
- Prefer `let` to `before(:each)` ivars.
- Define the module inline as a local; this avoids the need
  for the awkward `m = mod`.