test: handle missing V8 tests in n-api test

The N-API test testInstanceOf.js relies on several V8 test files
which may not exist in downloadable archives. If the files are
missing, this commit causes a warning to be emitted rather than
failing the test.

Refs: https://github.com/nodejs/node/issues/14113
Fixes: https://github.com/nodejs/node/issues/13344
Backport-PR-URL: https://github.com/nodejs/node/pull/19447
PR-URL: https://github.com/nodejs/node/pull/14123


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>