Make more tests using parsing for their input

[Morriar]

While a bit wasteful, writing the test input as a real RBI string rather than crafting in manually using the RBI classes is much easier to write/read.

Especially around nested scopes:

tree = parse_rbi(<<~RBI)
  class Foo
    class Bar
      class Baz; end
    end
  end
RBI

makes it easier to understand the scope hierarchy than:

tree = Tree.new
scope1 = Class.new("Foo")
scope2 = Class.new("Bar")
scope2 << Class.new("Baz")
scope1 << scope2
tree << scope1

[egiurleo]

This is waaaaay better