Add the test case for the "upload" API function
Add the test case for the "upload" API function

Was added the test for the metatool.core.upload() function, called with
encrypt=True argument. Function is expected to encrypt uploaded file
locally, and put proper "decryption_key" value, like the item to the
returned Json's content.

Simultaneously, whole test class for the "upload" API function was
refactored. Namely, repeated logic snippets was moved to the common
test fixtures.

Storj/metatool