enable updater tests to be run on their own

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/test/test_updater.rb

 abort if ARGV.include? "--skip-update"
 
 require 'testing_env'
+HOMEBREW_CELLAR.mkpath
 
 require 'extend/ARGV' # needs to be after test/unit to avoid conflict with OptionsParser
 ARGV.extend(HomebrewArgvExtension)