Restructured test fixtures.

Plugins are now stored in a separate repository, and have descriptive
names. Moved almost all fixture assertions into fixture SetUp/TearDown.
Reduced code duplication by inheriting from another class. Moved most
testing environment setup into fixture SetUp, so that tests share a more
common environment.

A few tests currently fail, it looks like there are issues with setting
load order.
Made initial load order / active state consistent.

It turns out that was the reason for the failed tests in the previous
commit.