vineyard: try to debug macos test failure

Follow-up to #230431

Author: carlocab

Formula/v/vineyard.rb

@@ -151,6 +151,8 @@ def install
     sleep 10
     sleep 10 if OS.mac? && Hardware::CPU.intel?
 
+    assert_path_exists testpath/"vineyard.sock"
+
     assert_equal("vineyard instance is: 0\n",
                  shell_output("#{testpath}/build/vineyard-test #{testpath}/vineyard.sock"))
   ensure