Extracting Ns._create_task() and creating unit test

Extracted the _create_task() to ease the testability of that code.
Introduced a unit test to cover that function.
Re-enabled unit tests and black now is mandatory.

Change-Id: I6b304b14b60b182dee6b274cf95228a81ddc2771
Signed-off-by: sousaedu <eduardo.sousa@canonical.com>