Attempt to fix race condition on test by depending on last compiled file

Author: José Valim

lib/mix/test/mix/umbrella_test.exs

@@ -144,7 +144,7 @@ defmodule Mix.UmbrellaTest do
 
         # Ensure we can measure a timestamp difference
         ensure_touched("../foo/lib/foo.ex",
-                       File.stat!("_build/dev/lib/foo/.compile.lock").mtime)
+                       File.stat!("_build/dev/lib/bar/.compile.lock").mtime)
 
         Mix.Task.run "compile"
         assert Mix.Tasks.Compile.Elixir.run([]) == :noop