Fix test failure for Sphinx >= 2.2.2

[D4N]

This fixes #465.

Sphinx changed the way it retrieves the CDomain in
sphinx-doc/sphinx@e42d546
to no longer use env.domaindata, but env.get_domain instead.

This breaks the mocks setup by breathe as they were implicitly only populatin
env.domaindata by invoking the constructor of the C and C++ domains (see
sphinx.domains.init: Domain.init). To properly populate the environment,
we need to make the mocked registry aware of the registered domains. Since we
only care about the C and C++ domain, we simply make it return both in
create_domains(), which registers them properly in the environment.

Note that this is unfortunately only a hack and a proper solution would be to port the testsuite to use sphinx' testing module instead, which in turn requires to use pytest instead of nosetests.

[t-b]

@D4N Nice! Your reasoning makes sense. I've checked locally that it works as advertised.

[vermeeren]

Thanks for the detailed analysis and the patches, all seems good to me.

I have created #473 as a potential follow-up issue for porting Breathe's tests using the information you found, which should be useful in the future.