TST: Fix generator tests to run
Closes #14244

This class inheriting from TestCase caused
the tests the yield fixture tests to not actually
run. The new output is

```
nosetests pandas/computation/tests/test_eval.py:TestTypeCasting
........................................
----------------------------------------------------------------------
Ran 40 tests in 0.264s

OK
```

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14245?src=pr) is 85.25% (diff: 100%)

> Merging [#14245](https://codecov.io/gh/pydata/pandas/pull/14245?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14245   diff @@
==========================================
  Files           140        140          
  Lines         50560      50560          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43101      43104     +3   
+ Misses         7459       7456     -3   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [db9dc65...56f6028](https://codecov.io/gh/pydata/pandas/compare/db9dc65a29667667c66f3d9759c4cb2048d85b34...56f6028fffa323908813ac36fd8d9174226038e3?src=pr)
 thanks @TomAugspurger 
