test: remove common.getServiceName()

Replace lightly-used services file parsing in favor of
confirming one of a small number of allowable values in service name
lookup tests.

In https://github.com/nodejs/node-v0.x-archive/issues/8047, it was
decided that this sort of service file parsing was superior to
hardcoding acceptable values, but I'm not convinced:

* No guarantee that the host uses /etc/services before, e.g., nscd.
* Increases complexity of tests without guaranteeing robustness.

I think that simply checking against a small set of expected values
may be a better solution. Ideally, there would also be a unit test that
used a test double for the appropriate `cares` function and confirms
that it is called with the correct parameters, but now we're getting way
ahead of ourselves.

PR-URL: https://github.com/nodejs/node/pull/6709


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>