test: add test-debug-protocol-execute

Add test for `Protocol` object in `_debugger` module. This test covers
some edge cases that fill some coverage gaps in our testing (such as the
"Unknown state" error).

PR-URL: https://github.com/nodejs/node/pull/8454


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>