tests: Set environment variable USER if not set

These two tests fail if the enivornment variable USER is not set:

./test/utils/user_spec.rb:6 # User should eq nil
./test/utils/user_spec.rb:22 # User#gui? when the current user is in a
console session gui? should equal true

Library/Homebrew/dev-cmd/tests.rb

@@ -49,6 +49,8 @@ module Homebrew
       ENV["HOMEBREW_TEST_GENERIC_OS"] = "1" if args.generic?
       ENV["HOMEBREW_TEST_ONLINE"] = "1" if args.online?
 
+      ENV["USER"] ||= Utils.popen_read("id", "-nu").chomp
+
       # Avoid local configuration messing with tests e.g. git being configured
       # to use GPG to sign by default
       ENV["HOME"] = "#{HOMEBREW_LIBRARY_PATH}/test"