Do not fail suite if there are no tests to run

This matters in umbrella apps where some apps may not
have Elixir tests.

Closes #8040

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

lib/mix/lib/mix/tasks/test.ex

@@ -407,7 +407,7 @@ defmodule Mix.Tasks.Test do
             Mix.shell().info("No stale tests")
 
           files == [] ->
-            raise_or_error_at_exit("There are no tests to run", opts)
+            Mix.shell().info("There are no tests to run")
 
           true ->
             message = "Paths given to `mix test` did not match any directory/file: "