Allow to reset async for ExUnit.Case by using it again (#9360)

lib/ex_unit/lib/ex_unit/case.ex

@@ -223,8 +223,6 @@ defmodule ExUnit.Case do
     end
 
     quote do
-      async = !!unquote(opts)[:async]
-
       unless Module.has_attribute?(__MODULE__, :ex_unit_tests) do
         tag_check =
           [:moduletag, :describetag, :tag]
@@ -249,11 +247,17 @@ defmodule ExUnit.Case do
 
         @before_compile ExUnit.Case
         @after_compile ExUnit.Case
-        @ex_unit_async async
+        @ex_unit_async false
         @ex_unit_describe nil
         use ExUnit.Callbacks
       end
 
+      async = unquote(opts)[:async]
+
+      if is_boolean(async) do
+        @ex_unit_async async
+      end
+
       import ExUnit.Callbacks
       import ExUnit.Assertions
       import ExUnit.Case, only: [describe: 2, test: 1, test: 2, test: 3]

lib/ex_unit/test/ex_unit/case_test.exs

@@ -113,3 +113,39 @@ defmodule ExUnit.CaseTest do
     end
   end
 end
+
+defmodule ExUnit.DoubleCaseTest1 do
+  use ExUnit.Case, async: true
+  use ExUnit.Case
+
+  test "async must be true", context do
+    assert context.async
+  end
+end
+
+defmodule ExUnit.DoubleCaseTest2 do
+  use ExUnit.Case, async: false
+  use ExUnit.Case
+
+  test "async must be false", context do
+    refute context.async
+  end
+end
+
+defmodule ExUnit.DoubleCaseTest3 do
+  use ExUnit.Case, async: true
+  use ExUnit.Case, async: false
+
+  test "async must be false", context do
+    refute context.async
+  end
+end
+
+defmodule ExUnit.DoubleCaseTest4 do
+  use ExUnit.Case
+  use ExUnit.Case, async: true
+
+  test "async must be true", context do
+    assert context.async
+  end
+end