test: add test for let bindings in for loops

072460265226c047369558b23e9ff2748965bf6c floats a patch on V8 that fixes
issue #9113 that would cause let bindings and continue statements in for
loops to not work properly.

This change adds a regression test that fails if that patch is not
properly floated, thus preventing us from not floating that patch after
future V8 upgrades.

Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
PR-URL: https://github.com/joyent/node/pull/23948