Fix: Move all fixtures to st2tests/fixtures

We have fixtures scattered everywhere and this PR attempts to consolidate the fixtures for maintainability. 

st2tests now has two folders: fixtures and resources.
fixtures - contain JSON files that represent st2 models. 
resources - other files that are required for testing. 

We have three history folders in fixtures. Another PR should address consolidating them. And history folders need to follow content_pack structure. 

Tests and lint pass.