Use mock'ed objects instead of manual swap

In tests, we can see a lot of code like:

obj2 = something()
obj_orig = self.engine_1.remote
self.engine_1.remote = obj2
# ... testings
self.engine_1.remote = obj_orig 

This is error prone as sometimes we do not restore the original object.
Also we should leverage the powerful unittest.mock.patch() function for those usages.

It will assure us less random bugs in some places.