test: allow for different nsswitch.conf settings

The motivation for this commit is that these two test fail on systems
that have different Name Service Switch configuration settings. A
concrete example of this is when using Red Hat Enterprise Linux (RHEL)
7.

If Name Service Switch is available on the operating system then it
might be configured differently (/etc/nsswitch.conf).
If the system is configured with no dns the error code will be
AI_AGAIN, but if there are more services after the dns entry, for
example some linux distributions skip a myhostname service by default
which would still produce the ENOTFOUND error.

This commit suggests checking for either ENOTFOUND or EAI_AGAIN to
accommodate systems like the ones described above. The references below
indicate that others have run, or are running, into this aswell.

Refs: https://github.com/nodejs/node/issues/12075
Refs: https://github.com/nodejs/help/issues/687
Refs: https://github.com/nodejs/node/issues/15825
PR-URL: https://github.com/nodejs/node/pull/16378


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>