use tmp_dir exclusively

Author: milmazz

test/ex_doc/formatter/markdown_test.exs

@@ -291,17 +291,15 @@ defmodule ExDoc.Formatter.MarkdownTest do
 
   describe "assets" do
     test "copies assets from source", %{tmp_dir: tmp_dir} = context do
-      dir_name = "test/tmp/markdown_assets/hello"
-      File.mkdir_p!(dir_name)
+      dir_name = Path.join(tmp_dir, "hello")
+      File.mkdir!(dir_name)
       dir_name |> Path.join("world") |> File.touch!()
 
       config = config(context, assets: %{dir_name => "assets"})
 
       generate(config)
 
       assert File.regular?(tmp_dir <> "/assets/world")
-    after
-      File.rm_rf!("test/tmp/markdown_assets")
     end
 
     test "does not override previous work from other formatters", %{tmp_dir: tmp_dir} = context do