Convert DMG test to spec.

Library/Homebrew/cask/test/cask/container/dmg_test.rb → Library/Homebrew/cask/spec/cask/container/dmg_spec.rb

-require "test_helper"
+require "spec_helper"
 
 describe Hbc::Container::Dmg do
-  describe "mount!" do
+  describe "#mount!" do
     it "does not store nil mounts for dmgs with extra data" do
       transmission = Hbc::CaskLoader.load_from_file(TEST_FIXTURE_DIR/"cask/Casks/local-transmission.rb")
 
@@ -13,7 +13,7 @@ describe Hbc::Container::Dmg do
 
       begin
         dmg.mount!
-        dmg.mounts.wont_include nil
+        expect(dmg.mounts).not_to include nil
       ensure
         dmg.eject!
       end