Isolated Tests for Backwards Compatibility Imports

[shawnbrown]

Datatest provides backwards compatibility via sub-packages in datatest.__past__. Importing a sub-package will modify datatest's behavior by applying monkey patches.

Care needs to be taken to isolate the side-effects of these imports because their default behavior is to apply global state changes to datatest, itself.

[shawnbrown]

This is done: c90080c

I decided to isolate these tests by putting them in separate files and adding test_past_subprocesses.py which runs each submodule test using subprocess.Popen.

Implementing module isolation with multiprocessing or threading modules looked to be needlessly complex.