test: specify global object for globals

Be explicit about using `global.externalizeString()` etc. in
test-fs-write instead of disabling the `no-undef` ESLint rule.

PR-URL: https://github.com/nodejs/node/pull/36498


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>