Add LastFm.Config test

It might seem redundant, but helps with refactoring down the line.

test/last_fm/config_test.exs

@@ -0,0 +1,20 @@
+defmodule LastFm.ConfigTest do
+  use ExUnit.Case, async: true
+
+  describe "resolve/1" do
+    test "reads data from application configuration" do
+      assert %LastFm.Config{
+               api: LastFm.APIBehaviourMock,
+               api_key: api_key,
+               user: user,
+               auto_refresh: false,
+               refresh_interval: refresh_interval
+             } =
+               LastFm.Config.resolve(:music_library)
+
+      assert is_binary(api_key)
+      assert is_binary(user)
+      assert is_integer(refresh_interval)
+    end
+  end
+end