TST/CLN: pare down the eval test suite

Current running time of the `eval` test suite (not including `query`) 

```
$ nosetests pandas/computation/tests/test_eval.py                                                   1 ↵
......../home/phillip/Documents/code/py/pandas/pandas/core/frame.py:3088: FutureWarning: TimeSeries broadcasting along DataFrame index by default is deprecated. Please use DataFrame.<op> to explicitly broadcast arithmetic operations along the index
  FutureWarning)
.........................................................................................S.......................................................................................................................
----------------------------------------------------------------------
Ran 217 tests in 190.685s

OK (SKIP=1)
```

New running time :speedboat: :

```
$ nosetests pandas/computation/tests/test_eval.py
......../home/phillip/Documents/code/py/pandas/pandas/core/frame.py:3088: FutureWarning: TimeSeries broadcasting along DataFrame index by default is deprecated. Please use DataFrame.<op> to explicitly broadcast arithmetic operations along the index
  FutureWarning)
.........................................................................................S.......................................................................................................................
----------------------------------------------------------------------
Ran 217 tests in 39.299s

OK (SKIP=1)
```