A little unittest refactoring
* Add a class decorator to generate tests when using a unittest.TestCase base class
* Add a TestCase subclass with setUp() and tearDown() that sets up
module parameter parsing
* Move test_safe_eval to use the class decorator and ModuleTestCase base
class
* Move testing of set_mode_if_different into its own file and separate
some test methods out so we get better errors and more coverage in
case of errors.
* Naming convention for test cases doesn't need to duplicate information
that's already in the file path.
Updated python module wrapper explode method to drop 'args' file next to module.
Both execute() and excommunicate() debug methods now pass the module args via file to enable debuggers that are picky about stdin.
Updated unit tests to use a context manager for masking/restoring default streams and argv.
Required some rewiring in inventory code to make sure we're using
the DataLoader class for some data file operations, which makes mocking
them much easier.
Also identified two corner cases not currently handled by the code, related
to inventory variable sources and which one "wins". Also noticed we weren't
properly merging variables from multiple group/host_var file locations
(inventory directory vs. playbook directory locations) so fixed as well.
