test: don't call process.exit() in gc tests

Rewrite the tests in test/gc so that they no longer call process.exit().
Instead they exit gracefully now.

PR-URL: https://github.com/nodejs/node/pull/11239


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>