sorbet: Bump most `test/rubocops/` to `typed: true` or higher

- By including `Rubocop::RSpec::ExpectOffense` and `CopHelper` and its
  methods in the RSpec shim, to allow Sorbet to find `expect_offense`
  and `inspect_source` etc., we can typecheck 94 of the tests for our
  custom RuboCop rules.