chore: run rubocop with rake test

Rakefile

@@ -3,12 +3,19 @@
 require "bundler/gem_tasks"
 require "rake/testtask"
 
-Rake::TestTask.new(:test) do |t|
+Rake::TestTask.new("test:lib") do |t|
     t.libs << "test"
     t.libs << "lib"
     t.test_files = FileList["test/**/*_test.rb"]
 end
 
+task "rubocop" do
+    raise "rubocop failed" unless system(ENV["RUBOCOP_CMD"] || "rubocop")
+end
+desc "Run tests"
+task "test" => "test:lib"
+task "test" => "rubocop"
+
 require "rake/extensiontask"
 
 task build: :compile