REF: directory for method-specific series/frame tests
Per the docstring in each of the new directories:

```
Test files dedicated to individual (stand-alone) DataFrame methods

Ideally these files/tests should correspond 1-to-1 with tests.series.methods

These may also present opportunities for sharing/de-duplicating test code. 
```

If this approach is approved, I'll then go through the remaining test files (in particular test_analytics) for things that have become "misc" piles.
