Fix mix test --only async:true/false

[SteffenDE]

This got accidentally broken in #13618.

@josevalim

[josevalim]

Instead of reading it here, maybe we can make it part of moduletag and that will just work? Then I think we can also remove it from __before_compile__ too.

[josevalim]

Another option is to set it inside ExUnit.Runner in run_module/prepare_tests.

[SteffenDE]

We validate that the module tags don't set any reserved tags. Async is in the list of reserved tags. If we remove it there and do a Module.put_attribute(module, :moduletag, [async: async?]) in __register__ it would work, but then users can also do

defmodule MyTest do
  use ExUnit.Case

  @moduletag async: true
end

I don't know if that's fine. The current change is basically how it was previously and I'm really no expert on ExUnit internals :)

[SteffenDE]

@josevalim should I send a separate PR for the tests I added here? Or do you think they‘re not really needed?

[josevalim]

@SteffenDE I am usually against fixtures, because they are expensive, but I tried to port your test to be fixture-less on the mix suite and failed. The thing I want to try next is to add test to test/ex_unit_test.exs, I think it is more doable, but I probably have time only after thursday. So if you want to give it a try and send a PR, i will gladly merge it :)