Fix bootstrap warning

Author: josevalim

lib/mix/lib/mix/compilers/test.ex

@@ -11,7 +11,8 @@ defmodule Mix.Compilers.Test do
     runtime_references: [],
     external: []
 
-  @compile {:no_warn_undefined, ExUnit.Server}
+  # Necessary to avoid warnings during bootstrap
+  @compile {:no_warn_undefined, ExUnit}
   @stale_manifest "compile.test_stale"
   @manifest_vsn 1