This patch adds a test for largefiles (creating, seeking, telling, etc.).
The test skips if there is no largefile support.

There is one further problem. The test basically involves creating a file
greater than 2GB and playing with it. On UN*X systems with sparse files this
is no problem. On Win64 (which I have heard *can* do sparse files, but not in
Python yet), however, >2GB space and a *long* time is required to run the
test. I don't think it is reasonable to turn this on by default... so here is
what I did.

I extended regrtest.py to accept the --have-resources switch. This sets
test_support.use_large_resources, which is checked in test_largefile.py.
By default 'use_large_resources' is false. On Win64, then, by default the
largefile test is skipped but can be run via the --have-resources switch to
regrtest.py or by running the test directly. This seems to me the Right
Thing.

The affected files are:
 Lib/test/regrtest.py
 Lib/test/test_support.py
 Lib/test/test_largefile.py (new)
 Lib/test/output/test_largefile (new)