The tests for time series aggregate functions don't inherit from AbstractAggregationTestCase.

This means we're not able to enforce the same kinds of invariants on them and only test them to the extent of any old function. They won't profit from test coverage implemented specifically for agg functions.

time series agg functions probably behave slightly differently from regular agg functions - but it'd still be nice to have them inherit from a common abstract test class, and generally that class should be a subclass of the AbstractAggregationTestCase unless there's good reasons against this.