(3) Simplify _C wrappers

This relates to issue #313.
How about offering two macros:

    TEST_GROUP_C_WRAPPER(group_name) // the original
    TEST_GROUP_C_SHORT_WRAPPER(group_name) // alternative

The name is just a first shot. Although, as I said before, I am not sure how one would actually make use of the attributes that _could_ be defined in the first version, when the actual tests and setup() / teardown() are in a C file. I would be hard-pressed to think up an example of that for the documentation :-)