sorbet: RBIs for more custom RSpec matchers like `not_to_output`

- Bumping some `test/cmd` files to `typed: true` revealed single errors
  where matchers like `not_to_output` were not recognized.
- These custom matchers are defined by `RSpec::Matchers.define` in
  various test files.
- So we have complete coverage of all custom matchers, not just
  predicates, search for all `RSpec::Matchers.define` calls and add them
  to the RBI file.