Convert uninstall_postflight test to spec.

Library/Homebrew/cask/test/cask/dsl/uninstall_postflight_test.rb → Library/Homebrew/cask/spec/cask/dsl/uninstall_postflight_spec.rb

-require "test_helper"
+require "spec_helper"
 
 describe Hbc::DSL::UninstallPostflight do
   let(:cask) { Hbc::CaskLoader.load_from_file(TEST_FIXTURE_DIR/"cask/Casks/basic-cask.rb") }