Add shared example for CLI command specs (#28165)

spec/support/examples/cli.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+shared_examples 'CLI Command' do
+  it 'configures Thor to exit on failure' do
+    expect(described_class.exit_on_failure?).to be true
+  end
+
+  it 'descends from the CLI base class' do
+    expect(described_class.new).to be_a(Mastodon::CLI::Base)
+  end
+end