Include tests in gem package

There is little overhead to this and it is easier to see how the gem works using just `bundle open` when these are in place.

munster.gemspec

@@ -23,7 +23,7 @@ Gem::Specification.new do |spec|
   spec.files = Dir.chdir(__dir__) do
     `git ls-files -z`.split("\x0").reject do |f|
       (File.expand_path(f) == __FILE__) ||
-        f.start_with?(*%w[bin/ test/ spec/ features/ .git .github appveyor Gemfile])
+        f.start_with?(*%w[bin/ .git .github appveyor Gemfile])
     end
   end
   spec.require_paths = ["lib"]