TST: Some nose generator tests not running
e.g. pandas/computation/tests/test_eval.py:TestTypeCasting 

``` bash
nosetests pandas/computation/tests/test_eval.py:TestTypeCasting -vs -x --with-id
#431 test_binop_typecasting (pandas.computation.tests.test_eval.TestTypeCasting) ... ok
```

The obvious difference is that `TestTypeCasting` inherits from `tm.TestCase`. I _thought_ nose was supposed to run these still, but apparently not.
EDIT: OK, nose [doesn't support those](http://nose.readthedocs.io/en/latest/writing_tests.html#test-classes):

> Test classes that do not descend from unittest.TestCase may also include generator methods and class-level fixtures

This came up as part of verifying that https://github.com/pydata/pandas/pull/13856 isn't dropping any tests.
