http2: simplify subsequent rstStream calls

Do not call destroy each time rstStream is called since the
first call (or receipt of rst frame) will always trigger
destroy. Expand existing test for this behaviour.

PR-URL: https://github.com/nodejs/node/pull/16753


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>