This makes std/node tests runnable from any directory by resolving the
testdata directory and files relative to the module directory resolved
from import.meta.url.
The promise approach still required permissions to be specified
at initialisation, rather than at request.
Using a Proxy instance solves this permission issue.
The Proxy instance approach also eliminates the need for the
await. Achieving direct compatibility with Node.js.
/ref pr #6392
/ref commit d16337cc9c
