test: remove destructor from node_test_fixture

This commit removes the destructor from node_test_fixture.h which calls
the TearDown function causing TearDown to be called twice. This also
allows us to remove the check of the platform_ in TearDown.

Also the Setup/TearDown functions in AliasBufferTest are removed as they
are not necessary.

PR-URL: https://github.com/nodejs/node/pull/18524


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