This commit consolidates the creation of the Aggregate struct, which was
previously handled differently in `prepare_one_select_plan` and
`resolve_aggregates`. That discrepancy caused inconsistent handling of
zero-argument aggregates.
The queries added in the new tests would previously trigger a panic.
When adding tests for extensions, please follow these guidelines:
Tests that verify the internal logic or behavior of a particular extension should go into cli_tests/extensions.py.
Tests that verify how extensions interact with the database engine, such as virtual table handling, should be written
in TCL (see vtab.test as an example).
To check which extensions are available in TCL, or to add a new one, refer to the tester.tcl file and look at the extension_map.
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