Add Test coverage on the handling of manager init

[elliotcmorris]

[fix] or [plan] We could do with some test coverage on the handling or manager init as its quite a mission-critical component. ie, dont set this here, and have fixtures that create a known configured environment, such that we can assert the behaviour with there is no default manager set, and when there is.

Originally posted by @foundrytom in https://github.com/OpenAssetIO/usdOpenAssetIOResolver/pull/16#discussion_r1093023309

Currently, initialising the manager (BAL) is done up front in an environment variable, and we don't have tests around reconfiguring/misconfiguring the manager. We should add some.