Add create export task tests

*Added create_export_task module:
-Added create_export_task test
-Updated variable names for clarity
-Updated test steps
-Updated placement of error validation
*Added create_export_task_negative module:
-Added attempt_duplicate_export_task test
-Added export_coalesced_snapshot test
-Updated test steps
-Updated module name
*Added create_export_task_positive module:
-Added export_same_image_two_different_containers test
-Updated test steps
-Updated module name
-Updated for loop

Change-Id: I899b1740477bc6fbda1ad9f62b063357c9319bad
Merge "Add create export task tests"