test: add missing initialization in test-assert

test-assert contains Boolean checks without initializing the Boolean to
false. It will be true thanks to previous tests in the file.

Block-scope all instances of `threw` so that side effects like this are
not an issue. Add missing initializations for `threw` in the tests
where it is missing.

PR-URL: https://github.com/nodejs/node/pull/11191


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>