test/cql-pytest: add simple tests for COUNT aggregation

[nyh]

This patch adds a few simple functional test for the COUNT aggregation feature, and in particular how count(*) differs from count(v), and how COUNT interacts with the GROUP BY operation.

3 of the 8 new tests are marked xfail, and reproduce two newly discovered issues:

Refs #12475: Global COUNT with empty IN results in an internal error
instead of the expected empty count.

Refs #12477: Combining COUNT with GROUP by results with empty results
in Cassandra, and one result with empty count in Scylla.

Signed-off-by: Nadav Har'El nyh@scylladb.com

[scylladb-promoter]

CI state FAILURE - https://jenkins.scylladb.com/job/releng/job/Scylla-CI/3743/

[nyh]

The new test caused Scylla to crash in a weird way:

ERROR 2023-01-09 18:22:56,247 [shard 0] forward_service - forward_aggregates::finalize(): operation cannot be completed due to invalid argument sizes. 

I rebased on new master hoping maybe the crash was something known and already fixed, but if not, I'll need to debug it. Maybe the "internal error" I saw (#12475) can in some circumstances cause a crash?

[scylladb-promoter]

CI state FAILURE - https://jenkins.scylladb.com/job/releng/job/Scylla-CI/3857/

[avikivity]

/cc @havaker @wmitros

[nyh]

Hmm, yes :-( It turns out that the test for #12475 (test_count_empty_in), which in dev mode causes "just" an internal error, in debug mode causes Scylla to crash. If I want to add these tests now I'll have to "skip" this test, but maybe it's better to just fix 12475 first, as it just got a little bit worse (it can be used to crash Scylla).

The failing test is simply running:

select count(*) from tbl where p in ()

This silly query, with an empty "in", causes Scylla to crash in debug mode. The error is
ERROR 2023-01-16 18:40:21,775 [shard 0] forward_service - forward_aggregates::finalize(): operation cannot be completed due to invalid argument sizes. this.aggrs.size(): 1 result.query_result.size(): 0.
You can see the full stack trace in https://jenkins.scylladb.com/job/releng/job/Scylla-CI/3857/testReport/junit/(root)/non-boost%20tests/cql_pytest_test_count_debug_1/

[nyh]

I'm closing PR and will open a new one which will fix one of the bugs that the test found - that cause Scylla to crash in debug mode (because of an "on_internal_error") and the CI to fail.

[nyh]

Opened #12715 to replace this one.