test: s/ASSERT/assert/

This commit addresses an inconsistency with eight tests. These
tests use the assert module, but named the variable ASSERT. This
goes against the project's typical coding style, and negatively
impacts global find and replace updates.

PR-URL: https://github.com/nodejs/node/pull/10544


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Jackson Tian <shyvo1987@gmail.com>
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>