suppress warnings when testing deprecated interfaces

[Martin Pool]

> > The test suite prints all deprecation warnings on the error output. But isn't
> > it supposed to test deprecated interfaces as long as they are kept around?
>
> Whenever an interface becomes deprecated, none of the core code should
> use it. Printing deprecation warnings during test runs is a good way to
> catch that when it does happen.
>
> It's hard to distinguish between tests of deprecated functions and tests
> of undeprecated functions that happen to call deprecated functions,
> which means we're getting error output even for deprecations warnings we
> expect.
>
> Perhaps we need a decorator to suppress expected deprecations warnings
> for test cases that exercise deprecated interfaces.

Something like

def test_deprecated_function():
  warnings.filterwarnings('ignore', 'frob_thing is deprecated.*')
  try:
    frob_thing(3)
  finally:
    unfilter_it()

On a brief look at the warnings module there isn't a clear way to remove
just the one specific filter we had put in place, but it can probably be
done, by brute force if necessary.

Also we should probably make a UserWarning subclass for deprecations.

Or perhaps more simply, the test case for a class can just filter out
all deprecation warnings for calls to deprecated methods of the class
directly from the test:

  warnings.filterwarnings('ignore', 'SomeClass.* is deprecated',
                          DeprecationWarning, 'test_some_class')

and then not worry about pulling it out. I have not tested yet if this
works.

[John A Meinel (jameinel)]

Aaron added the test suite function TestCase.callDeprecated(), which will call a deprecated function, and assert that it does, in fact, cause deprecation warnings, but will prevent them from being written to the screen.