CLN: Split up Boolean array tests
Apologies for the size of this PR, but it looks like the ExtensionArray tests sometimes have their own directories by type, and other times all tests for the type are in a single file (e.g., https://github.com/pandas-dev/pandas/tree/master/pandas/tests/arrays/categorical vs. https://github.com/pandas-dev/pandas/blob/master/pandas/tests/arrays/test_boolean.py). I think the "whole directory for type" structure makes it easier to find and add new tests, so I'm doing that here for the Boolean tests. There shouldn't be any changes other than moving things around.