test: improve coverage of the util module

Add tests for untested branches and statements.
Change assert.equal to assert.strictEqual for consistency.

PR-URL: https://github.com/nodejs/node/pull/8633


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>