Don't delete /tmp before generator tests

[pablobm]

I think I finally discovered why the directory /tmp was being deleted, a behaviour that has been confusing us lately. (See #1731, #1726, #1695).

The issue appears to be with the generator specs. These load this configuration when they run:

administrate/spec/support/generator_spec_helpers.rb

Lines 54 to 61 in 520a544

	RSpec.configure do |config|
	config.include GeneratorSpecHelpers
	config.before(:example, :generator) do
	destination File.expand_path("../../../tmp", __FILE__)
	prepare_destination
	end
	end

In this code, destination and prepare_destination are helpers provided by Rails to assist when testing generators. See https://api.rubyonrails.org/v3.2.2/classes/Rails/Generators/TestCase.html

Turns out that prepare_destination prepares the destination by... rimraffing it. See: https://github.com/rails/rails/blob/557014d45aeed82ed1e79b83ab59697392a1a5de/railties/lib/rails/generators/test_case.rb#L235

So if we set /tmp as destination, it will be destroyed ahead of each generator spec example.

To fix this, I'm telling it to use a specific subdirectory that can be deleted at will, avoiding further harm.

[nickcharlton]

Oh! That explains a lot.