This patch contains a first step towards classifying CPython tests into
language tests and implementation-details tests.  The patch is against
the 2.7 trunk's test_descr.py.  It is derived from a similar patch that
we wrote for the 2.5's test_descr.py in order to make it pass on PyPy.

The main new feature introduced here is a couple of helpers in
test_support - see comments and docstrings in the patch.  The main ones
are "check_impl_detail", which is a flag which is False on non-CPython
hosts; and "impl_detail", a decorator to skip whole functions based on
the "check_impl_detail" flag.

If this patch is accepted, then we plan to port many more of PyPy's
patches for core tests, as a step towards helping non-CPython
implementations to obtain a good test suite.