Convert `staged` test to spec.

Library/Homebrew/cask/test/cask/staged_test.rb → Library/Homebrew/cask/spec/cask/staged_spec.rb

-require "test_helper"
+require "spec_helper"
 
 # TODO: this test should be named after the corresponding class, once
 #       that class is abstracted from installer.rb.  It makes little sense
 #       to be invoking bundle_identifier off of the installer instance.
 describe "Operations on staged Casks" do
   describe "bundle ID" do
+    let(:cask) { Hbc::CaskLoader.load_from_file(TEST_FIXTURE_DIR/"cask/Casks/local-transmission.rb") }
+    let(:installer) { Hbc::Installer.new(cask) }
     it "fetches the bundle ID from a staged cask" do
-      transmission_cask = Hbc::CaskLoader.load_from_file(TEST_FIXTURE_DIR/"cask/Casks/local-transmission.rb")
-      tr_installer = Hbc::Installer.new(transmission_cask)
-
       shutup do
-        tr_installer.install
+        installer.install
       end
-      tr_installer.bundle_identifier.must_equal("org.m0k.transmission")
+
+      expect(installer.bundle_identifier).to eq("org.m0k.transmission")
     end
   end
 end