Fix: Move all fixtures to st2tests/fixtures
Cleanup fixtures in st2tests/fixtures.

  * Fix: Consolidate history fixtures into one.
  * Fix: Remove unused fixtures.
Merge pull request #771 from StackStorm/STORM-582/cleanup_fixtures

RFR: Cleanup fixtures in st2tests.
Merge pull request #770 from StackStorm/STORM-582/fixtures_st2tests

Fix: Move all fixtures to st2tests/fixtures