Improve DSL test for custom app_root

[andyw8]

Motivation

Address feedback in https://github.com/Shopify/tapioca/pull/1220/files#r993606253

Implementation

Write the model files directly rather than having to move them.

Tests

n/a

[andyw8]

There seems to be an unrelated failing test for Rails 6.1:

      it compiles signatures and structs in source files              FAIL (0.87s)
        --- expected
        +++ actual
        @@ -143,7 +143,8 @@
           sig { returns(T::Array[::Integer]) }
           def foo; end
         
        -  def foo=(_arg0); end
        +  sig { params(foo: T::Array[::Integer]).returns(T::Array[::Integer]) }
        +  def foo=(foo); end
         
           sig { override.returns(::Integer) }
           def something; end
        /home/runner/work/tapioca/tapioca/spec/tapioca/gem/pipeline_spec.rb:3008:in `block (2 levels) in <class:PipelineSpec>'

[KaanOzkan]

I don't know why the error is happening despite the directory being purged same as before. We can keep the existing solution if it requires a long investigation.

[andyw8]

Fix in progress for CI: #1226

[vinistock]

The issue is because there's no Gemfile.lock for the Ruby 2.7 gemfile. This makes it so that we always install the latest versions when running CI and the newest Sorbet version added runtime checks to attribute writers, which changes the expected output. This is also failing in main, so we should fix that, but this PR looks good.