test: Fix missing propagation of return status when calling pytest in ./test.py

[svalat]

Big ooops, forgot to return the pytest status to propagate it as it does not throw an exception.

Also fix issues to make the test suite green again :

• Access to a GPU cell from CPU in previous communication refactoring.
• Missing call to a python function for a test
• Wrong array used in a previous refactoring about MPI communications.
• Fix a warning due to unused variable

Python call in testme.json

In order to fix the missing call the MakeNumpyFile() function in the test test/utils/lookupTable I added in this PR a new semantic in the test suite to be able to call a Python function from the testme.json test definition.

It is simply two new fields which can be used :

"default": {
"callPyFunctionBefore": "testmelib.py:callMeAtStart",
"callPyFunctionAfter": "testmelib.py:callMeAtEnd"
}

It will call callMeAtStart before running the test and callMeAtEnd at the end of the test, the two functions beeing defined in testmelib.py.

Note: the documentation has been updated.