Clear ENV["XDG_CONFIG_HOME"] to avoid loading user-defined irbrc in test

[tompng]

TestIRB::ConfigValidationTest#setup forgot to clear ENV['XDG_CONFIG_HOME']

With these environment variables, rake test fails.

echo "puts 'IRBRC'; exit" > /path/to/dummy_home/env_irbrc
echo "puts 'HOME'; exit" > /path/to/dummy_home/.irbrc
echo "puts 'XDG'; exit" > /path/to/dummy_home/.config/irb/irbrc
IRBRC=/path/to/dummy_home/env_irbrc HOME=/path/to/dummy_home/ XDG_CONFIG_HOME=/path/to/dummy_home/.config rake test

Test passes with normal irbrc content maybe with Error loading RC file warning message, but it's better not to read user-defined irbrc file.