Refac tests

Closes #83 (closed) I recon it's a big diff but is mostly reorganization.

• Write tmp files in /dev/shm (?)
• Grouptest_* in funcs that makes more sense (e.g. test_app_*, test_output, test_slicer, test_summarize_*)
• Split single operations func into test_add, test_sub, test_mult, test_div
• Less reports of pipeline tests in XML (6 tests now), but print every failed test case and success of each block write or test shape
• Use WARNING logger level for pipelines test (since the debug output for 700 apps is not readable anyway)

With these new func names it's more easy to use pytest for a subset of tests. For example :
python -m pytest -vv -k test_summarize tests/test_core.py

I ended giving up with my fixture idea for now (I tried a get_input_obj func to generate a new Input object for each unit test) : it was really slow since the image is remote, and it doesn't look like something we should worry about for now.
But if we ever have strange tests results we may need to re-think about it...