Add back test skip for platforms without USR2

test/test_preserve_bundler_env.rb

@@ -9,6 +9,8 @@ class TestPreserveBundlerEnv < TestIntegration
 
   # It does not wipe out BUNDLE_GEMFILE et al
   def test_usr2_restart_preserves_bundler_environment
+    skip_unless_signal_exist? :USR2
+
     @tcp_port = UniquePort.call
     env = {
       # Intentionally set this to something we wish to keep intact on restarts