Formatter fails with no configured tests

[FrancisMurillo]

Just a minor issue. When running mix test with no tests configured. The formatter has the error message:

[error] GenServer #PID<0.546.0> terminating
** (KeyError) key :test not found in: %{}
    (excheck) lib/excheck/formatter.ex:66: ExCheck.Formatter.update_tests_counter/1
    (excheck) lib/excheck/formatter.ex:28: ExCheck.Formatter.handle_cast/2
    (stdlib) gen_server.erl:601: :gen_server.try_dispatch/4
    (stdlib) gen_server.erl:683: :gen_server.handle_msg/5
    (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Last message: {:"$gen_cast", {:suite_finished, 36754, nil}}
State: %{colors: [enabled: true], failure_counter: 0, invalid_counter: 0, seed: 627868, skipped_counter: 0, test_counter: %{}, trace: false, width: 80}

This occured when I was smoke testing the minimum configuration before adding tests. Not fatal but it did give me a tiny spook if I misconfigured anything with this library.