Add spec for ShortenerGenerator

spec/generators/shortener_generator_spec.rb

@@ -0,0 +1,10 @@
+require "spec_helper"
+require File.expand_path("../../../lib/generators/shortener/shortener_generator.rb", __FILE__)
+
+RSpec.describe ShortenerGenerator do
+  describe ".next_migration_number" do
+    it "returns the next migration number" do
+      expect { described_class.next_migration_number(".") }.not_to raise_error
+    end
+  end
+end