test: add test for child_process.execFile()

While `child_process.execFile()` gets called in places in the test
suite, there are no explicit test for it and there are parts of the
implementation that are not covered by tests. This adds a minimal test
that increases (but does not complete) coverage for the implementation.

PR-URL: https://github.com/nodejs/node/pull/11929


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>