test: cleanup test-dgram-error-message-address

* var -> const
* assert.equal() -> assert.strictEqual()
* assert.notEqual() -> assert.notStrictEqual()

Fixes: https://github.com/nodejs/node/issues/8925
PR-URL: https://github.com/nodejs/node/pull/8938


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>