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.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
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