Use different method for getting ext_suffix

```
ext_suffix = get_config_var("EXT_SUFFIX") or get_config_var("SO")
```

because `get_config_var("SO")` returns None in Python 3.4.0a4 because the "SO"
variable is deprecated and "EXT_SUFFIX" is the new way to get this information
(see: http://bugs.python.org/issue19555)

This fixes `TypeError: Can't convert 'NoneType' object to str implicitly`
errors when running the tests on Python 3.4.0a4.
Refactor 3 places handling fastmath ImportError

so that they call `Crypto.SelfTest.st_common.handle_fastmath_import_error`,
thereby eliminiating duplicate code.
Fix handle_fastmath_import_error (broken due to incorrect path in the previous commit)

Tested on py21-py33 by force-uninstalling libgmp10 after building.