test: use ES2017 syntax in test-fs-open-*

Update test-fs-open-flags to take advantage of destructuring and default
values.

Update test-fs-open-mode-mask to use a ternary to make use of a
constant possible.

PR-URL: https://github.com/nodejs/node/pull/23031


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>