test_cache():  Muck with Python's warnings module.

The test didn't work if run more than once (e.g., via Zope3's
test.py's "-N <n>" argument, or via ZODB's test.py's "-L"
argument), because the warnings module suppresses warnings
after the first from a single line.  Made the warnings module
report repeats of DeprecationWarnings for the duration of
the test.