TESTS: Remove all direct references to descriptor

Now all descriptors tests have to be done through the manager.
Part of #36.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>
TESTS: No longer making copy of extracted descriptor

Since the manager can non longer be spared in order to retrieve
descriptors, we might as well directly read the values within its
internal memory.
Part of #36.

Signed-off-by: Gama <misept.dieseneuf@gmail.com>