test: make cctest fixture use node::NewIsolate

This commit updates the gtest fixture to use node::NewIsolate instead of
creating a new V8 Isolate using v8::Isolate::New.

The motivation for this is that without calling node::NewIsolate the
various callbacks set on the isolate, for example AddMessageListener,
SetFatalErrorHandler etc, would not get set. I don't think this is the
expected behaviour and I ran into this when writing a new cctest.

PR-URL: https://github.com/nodejs/node/pull/21419


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>