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.
Personally, I don't find that this makes it easier to find the tests, though ... (with this I would need to look in multiple files of which I don't know the name)

Now, I assume that if we keep adding tests here, splitting it becomes unavoidable. 

The question might then also be: where to put the line on what should be tested here? 
For example, @jbrockmendel mentioned parametrizing the op tests to also test with series and dataframe. But then those tests maybe rather belong in one of the other test modules that are more specific, like tests/arithmetic, tests/reductions/, ...?
 > mentioned parametrizing the op tests to also test with series and dataframe. But then those tests maybe rather belong in one of the other test modules that are more specific, like tests/arithmetic, tests/reductions/, ...?

Yes, if that was unclear: if the arithmetic/reduction tests can/should be parametrized over EA/Series/DataFrame, then they likely should be moved to tests.arithmetic or tests.reductions etc.  (the situation is slightly complicated by the fact that those tests are also parametrized over Index which these can't be until ExtensionIndex is in operation) Curious if @jreback has any preferences on the layout of these tests? once a file gets > 500 lines, splitting into sub-modules is really needed. we have already split most other tests, so this is fine.