Added missing import for `platform` in tests/support.py for os x.

`platform` was not included in `test/support.py`, leading to the following errors in OS X:

```
args = (<test_selectors.PollSelectorTestCase testMethod=test_above_fd_setsize>,), kw = {}

    @functools.wraps(func)
    def wrapper(*args, **kw):
        if sys.platform == 'darwin':
>           version_txt = platform.mac_ver()[0]
E           NameError: name 'platform' is not defined

tests/support.py:19: NameError
```