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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

test child_process