add wesnoth-test text domain to a unit test which was missing it
fixup test utils to accomodate recent test directory change

Previously, the C++ unit tests which used the test utils
game config manager object would load things in /data/test/,
and not all of /data/. Presumably this was for efficiency
of the C++ unit tests.

I didn't realize this when working on the wml unit tests branch.
In that branch, I added all of the WML unit test macros and
scenarios to data/test/, and found that I couldn't load them,
since /data/_main.cfg had the {/test/_main.cfg} line commented
out. I uncommented this and made some other changes in
/data/test/ so that loading wml unit tests would work.

However, it made the C++ unit tests not happy.
This change makes them not create errors, but it also includes
all of core when they run, which may not have been intended,
although I don't see that it causes problems.

I may fix this all up soon, by making a different environment
for C++ and WML unit tests. (In fact this already existed,
and I have now destroyed the distinction.) Putting WML
tests in their own directory different from the C++ tests
may be the simplest fix, that will preserve the environment
of the old C++ tests while giving a more complete environment
where the WML unit tests may run.
Merge branch 'master' of https://github.com/wesnoth/wesnoth