test,util: add missing tests and conditions

1) Add missing unit tests by `ucs-2` in different kinds of cases.
2) Add missing unit tests by `usc-2` in different kinds of cases.
3) Fix a bug：We cannot find `ucs-2` in `case 5`'s `if` condition after
`toLowerCase()`

PR-URL: https://github.com/nodejs/node/pull/21455


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>