Add Spec for Usage

spec/app_store_connect/client/usage_spec.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+RSpec.describe AppStoreConnect::Client::Usage do
+  describe '#track' do
+    it 'should track usage w/ mixpanel' do
+      expect_any_instance_of(Mixpanel::Tracker)
+        .to receive(:track)
+        .with(String, 'usage')
+
+      described_class.new.track
+    end
+  end
+end