Slight improvement to test_execution.py
Refactor logic related to creating tools for unit tests.
So it can be reused by tool actions unit test.
Create unit test for some simple DefaultToolAction functionality.
Want to refactor some stuff around in DefaultToolAction so can be reused when dealing with dataset collections downstream in https://github.com/jmchilton/galaxy-central/tree/collections_1 - so creating unit tests to ensure functionality is not changing.

This changeset also reworks test_execution.py moving more stuff to test/unit/tools_support.py to share between test files.