test: make sure that the default naming scheme name maps back to itself

test: make sure that the default naming scheme name maps back to itself

We were testing the that C constant is defined, but we weren't actually testing
that the string name maps back to itself. This would catch the issue fixed by
the grandparent commit.

The test for the default name is moved to the test file to keep the tests
together. The define is renamed to not have "_TEST" in the name. The issue here
is complicated by the fact that we allow downstreams to inject additional
fields, so we don't know the name of the default scheme if it not set with
-Ddefault-net-naming-scheme=, so _DEFAULT_NET_NAMING_SCHEME[_TEST] is not
defined in all cases, but at least in principle it could be used in other
places. If it exists, it is fully valid.