Encode current behavior of helpers parsing

[Morriar]

Currently, the only Sorbet helpers we parse as Helper instance are abstract!, interface! and sealed!. Other helpers are parsed as Send instance:

tree = parse_rbi(<<~RBI)
  abstract!
  foo!
RBI

puts tree.nodes # => [<Helper @name="abstract">, <Send @name="foo!">]

Though you can create different Helper instances using the API:

Helper.new("foo!")

I'm not sure this is the right behavior but let's at least encode that this is expected in the tests.