test: use v8 Default Allocator in cctest fixture

This commit updates the node_test_fixture to use
v8::ArrayBuffer::Allocator::NewDefaultAllocator() and removes the custom
allocator.

PR-URL: https://github.com/nodejs/node/pull/17366


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>