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test_builtin.py is now updated to test more error
situations. This increases the coverage of bltinmodule.c
from 75.13% to 92.20%, and it actually revealed one or two
potential bugs:
http://www.python.org/sf/665761 and
http://www.python.org/sf/665835

I'm not 100% sure that test_intern() and test_execfile() do
the right thing.

I'm not sure, whether the test script should check for
undocumented implementation artefacts, like:

a = 1
self.assert_(min(a, 1L) is a)

but in this way at least we get notified if something is
changed unintentionally.