Do not generate file fixture methods

[sambostock]

Motivation

Tapioca currently erroneously generates methods for file fixtures.

For example if one were to create the following file in a Rails app

# test/fixtures/files/greetings.yml
Hello: there

Tapioca would generate the following RBI:

class ActiveSupport::TestCase
  sig { params(fixture_names: T.any(String, Symbol)).returns(T.untyped) }
  def files_greetings(*fixture_names); end
end

But that method does not exist.

Implementation

By defining file_fixture_path, ActiveRecord::TestFixtures skips over the directory.

See https://github.com/rails/rails/blob/7c70791470fc517deb7c640bead9f1b47efb5539/activerecord/lib/active_record/test_fixtures.rb#L46

Tests

A test is which includes a file fixture and asserts no method is defined for it.

[egiurleo]

This looks good to me! Let's wait for another review from Ruby DX before merging.