Add spec for the CLI version task

This causes all of the cli classes to be loaded, which is a big win
in code coverage LOC in exchange for this trivial spec addition.

spec/lib/mastodon/cli_spec.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+require 'cli'
+
+describe Mastodon::CLI do
+  describe 'version' do
+    it 'returns the Mastodon version' do
+      expect { described_class.new.invoke(:version) }.to output(
+        a_string_including(Mastodon::Version.to_s)
+      ).to_stdout
+    end
+  end
+end