test: fixing AliasedBuffer tests to enter Isolate

AliasedBuffer tests weren't creating an v8::Isolate::Scope, and this
had negative impact on the node-chakracore branch, where expectation
is an Isoalte has been Enter()'d before being used.

PR-URL: https://github.com/nodejs/node/pull/15536


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>