test: include all stdio strings for fork()

test-child-process-fork-stdio-string-variant was only testing 'pipe' for
its `stdio` value. Add `inherit` and `ignore`.

Also added a `common.mustCall()` to verify that the `message` event is
triggered.

PR-URL: https://github.com/nodejs/node/pull/11783


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>