Make test_nemesis independent from Nemesis disruption methods

[pehala]

Currently, the test_nemesis.py searches through all disruptions in Nemesis class, meaning even production ones.
This makes it hard to understand tests and does mean that the actual tests are dependant on the production disruption methods.

We should rethink how we test nemesis execution/discovery process and make it independent so have higher confidence in our tests and make them simpler.

One of the solution to this problerm would be to rewrite tests after #10502, which enables us to separate or even search methods in different class tree (i.e. create our own Nemesis class tree).