TST: Clean test_format.py
Mostly just stylistic changes. The only substantive one is the test now on line 56 (`has_horizontally_truncated_repr`). I can't figure what the test was supposed to be doing. Suggestions?

Original Test:

``` python
def has_horizontally_truncated_repr(df):
    try: # Check header row
        fst_line = np.array(repr(df).splitlines()[0].split())
        cand_col = np.where(fst_line=='...')[0][0]
    except:
        return False
    # Make sure each row has this ... in the same place
    r = repr(df)
    for ix,l in enumerate(r.splitlines()):
         if not r.split()[cand_col] == '...':
             return False
    return True
```

New Test:

``` python
def has_horizontally_truncated_repr(df):
    try: # Check header row
        fst_line = np.array(repr(df).splitlines()[0].split())
        cand_col = np.where(fst_line=='...')[0][0]
    except:
        return False
    # Make sure each row has this ... in the same place
    return repr(df).split()[cand_col] == "..."
```
