TST/CLN: Split out some to_string tests
test_format.py is over 3000 lines long so breaking some (but not all) to_string-specific tests into a separate test_to_string.py file (we already have test_to_html.py, test_to_latex.py, and test_to_markdown.py)
> looks fine. can you confirm we have same number of tests before & after here.

master
```
pandas/tests/io/formats/test_format.py .............................................................................................................................................................. [ 67%]
.............................................................................                                                                                                                         [100%]

============================================================================================ 235 passed in 4.39s ============================================================================================
```

branch
```
pandas/tests/io/formats/test_format.py .............................................................................................................................................................. [ 78%]
...........................................                                                                                                                                                           [100%]

============================================================================================ 201 passed in 4.21s ============================================================================================
```

```
pandas/tests/io/formats/test_to_string.py ..................................                                                                                                                          [100%]

============================================================================================ 34 passed in 0.42s =============================================================================================
```
 thanks @dsaxton 