test: improve domain-top-level-error-handler-throw

Use assert.strictEqual instead of assert.equal.

PR-URL: https://github.com/nodejs/node/pull/9950


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>