test: add block scoping to test-assert-deep

Add block scoping to test-assert-deep.js to reduce likelihood of one
test case having side effects that affect another test case.

PR-URL: https://github.com/nodejs/node/pull/16532


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>