TST: Mock clipboard IO
No idea if this will work on not.
This failed.

Unless anyone objects, I'm going to just mock the actual clipboard layer part of things. yeah the clipboard tests are annoying  Hello @TomAugspurger! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/tests/io/test_clipboard.py`](https://github.com/TomAugspurger/pandas/blob/f1ed15bbb9cc7c92c5ebd2ac2ad6d5eaf4148f2f/pandas/tests/io/test_clipboard.py) !



##### Comment last updated on September 16, 2018 at 11:57 Hours UTC OK, I think we're still testing things, but would appreciate a close look at the mocks.

Basic idea is to replace `get_clipboard` / `set_clipboard` with a simple dict mapping test_id -> data.

Unfortunately, we had some tests in `test_clipboard` that directly used `clipboard_get` and `clipboard_set`, so I had to directly manipulate the dict for those tests... Moved the mock clipboard to be function-scope, rather than global. @jreback any objections to merging? I wouldn't usually move this quickly but

a.) these tests have been failing forever
b.) It's kinda blocking https://github.com/pandas-dev/pandas/pull/22699 sure