test: add missing assert.deepEqual() test case

None of the existing tests checked for the situation where
`assert.deepEqual()` receives two objects that have the same number of
keys but different key names. Therefore, line 242 of `lib/assert.js` was
not being exercised by any tests.

This change adds the missing test case.

PR-URL: https://github.com/nodejs/node/pull/8152


Reviewed-By: jasnell - James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>