Nik Everett 956ae93fdb Better mimick real reduction in test
This changes has `InternalAggregationTestCase` performs partial
reductions to better mimick `InternalAggregations#reduce`. In
particular, this fixes a bug when we'd attempt a partial reduction of
aggregations that were entirely unmapped but use a mapped aggregation as
the "lead" of the reduction. This was possible in the test but isn't
possible in production. The reduction "lead" is unmapped if all agg
results being reduced are unmapped.

Closes #65118
2020-11-20 13:10:34 -05:00
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