test: complete coverage for lib/assert.js

6481c93a modified `lib/assert.js` and added some tests for new
functionality, but left a single line uncovered by tests. This adds a
test that covers the currently-uncovered line (which is the final
`return` statement in `setHasSimilarElement()`).`

PR-URL: https://github.com/nodejs/node/pull/12239


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>