Convert NestedContainer test to spec.

Library/Homebrew/cask/test/cask/artifact/nested_container_test.rb → Library/Homebrew/cask/spec/cask/artifact/nested_container_spec.rb

-require "test_helper"
+require "spec_helper"
 
 describe Hbc::Artifact::NestedContainer do
   describe "install" do
     it "extracts the specified paths as containers" do
       cask = Hbc::CaskLoader.load_from_file(TEST_FIXTURE_DIR/"cask/Casks/nested-app.rb").tap do |c|
-        TestHelper.install_without_artifacts(c)
+        InstallHelper.install_without_artifacts(c)
       end
 
       shutup do
         Hbc::Artifact::NestedContainer.new(cask).install_phase
       end
 
-      cask.staged_path.join("MyNestedApp.app").must_be :directory?
+      expect(cask.staged_path.join("MyNestedApp.app")).to be_a_directory
     end
   end
 end