tests: Allow use of deprecated API in unit tests

We need to test that the behaviour of deprecated API doesn’t change
across the versions of libgdata, which requires it to be accessible in
the headers, and hence LIBGDATA_DISABLE_DEPRECATED cannot be defined.

All deprecated API is marked with G_GNUC_DEPRECATED anyway, and the
tests can explicitly disable this with G_GNUC_BEGIN_IGNORE_DEPRECATIONS.

https://bugzilla.gnome.org/show_bug.cgi?id=687597