Add shared example for CLI command specs (#28165)

spec/lib/mastodon/cli/main_spec.rb

@@ -4,11 +4,7 @@ require 'rails_helper'
 require 'mastodon/cli/main'
 
 describe Mastodon::CLI::Main do
-  describe '.exit_on_failure?' do
-    it 'returns true' do
-      expect(described_class.exit_on_failure?).to be true
-    end
-  end
+  it_behaves_like 'CLI Command'
 
   describe 'version' do
     it 'returns the Mastodon version' do