FATE tests organization/maintainability improvement

**Is your feature request related to a problem? Please describe.**
The fate test package contains several custom test FATE objects (currently: FastFate, FlakyFate, SlowFateSplit) and several custom test Manager objects (currently: FlakyFateManager, SlowFateSplitManager).

**Describe the solution you'd like**
Would be clearer and more maintainable if these had a super class like CustomFate and CustomFateManager. Could also potentially move all these classes into their own sub package within the fate test package.

**Describe alternatives you've considered**
Could leave as is.

Start by locating the FATE test package and the listed classes: FastFate, FlakyFate, SlowFateSplit, FlakyFateManager, and SlowFateSplitManager. Review their shared behavior and existing tests before deciding whether CustomFate and CustomFateManager or a subpackage is appropriate; done means the organization is clearer and the FATE tests still pass.

Written by the indexing model from the issue text.