Merge pull request #5776 from sjackman/tests-user

tests: Set environment variable USER if not set

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"] ||= system_command!("id", args: ["-nu"]).stdout.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"