Formalize some patterns in cli specs (#28255)

spec/lib/mastodon/cli/domains_spec.rb

@@ -4,20 +4,26 @@ require 'rails_helper'
 require 'mastodon/cli/domains'
 
 describe Mastodon::CLI::Domains do
+  subject { cli.invoke(action, arguments, options) }
+
   let(:cli) { described_class.new }
+  let(:arguments) { [] }
+  let(:options) { {} }
 
   it_behaves_like 'CLI Command'
 
   describe '#purge' do
+    let(:action) { :purge }
+
     context 'with accounts from the domain' do
-      let(:options) { {} }
       let(:domain) { 'host.example' }
       let!(:account) { Fabricate(:account, domain: domain) }
+      let(:arguments) { [domain] }
 
       it 'removes the account' do
-        expect { cli.invoke(:purge, [domain], options) }.to output(
-          a_string_including('Removed 1 accounts')
-        ).to_stdout
+        expect { subject }
+          .to output_results('Removed 1 accounts')
+
         expect { account.reload }.to raise_error(ActiveRecord::RecordNotFound)
       end
     end