test: improve code coverage for i18n

Coverage report for src/node_i18n.cc shows that the
Has() method is not covered. This test adds coverage
for that method.

PR-URL: https://github.com/nodejs/node/pull/25428


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>