test: Add a test for HTTP/3 graceful shutdown
test: Add a test for HTTP/3 graceful shutdown

This commit adds a test case for HTTP/3 graceful shutdown, which
confirms that a client actually receives GOAWAY frame upon
server shutdown.

devnexen/h2o,h2o/h2o,devnexen/h2o,yannick/h2o,cwyang/h2o,yannick/h2o,yannick/h2o,devnexen/h2o,yannick/h2o,cwyang/h2o,yannick/h2o,cwyang/h2o,h2o/h2o,h2o/h2o,yannick/h2o,cwyang/h2o,devnexen/h2o,devnexen/h2o,cwyang/h2o,h2o/h2o,yannick/h2o,devnexen/h2o,yannick/h2o,yannick/h2o,devnexen/h2o,devnexen/h2o,cwyang/h2o,cwyang/h2o,devnexen/h2o,h2o/h2o,h2o/h2o,cwyang/h2o,h2o/h2o,h2o/h2o,h2o/h2o