Improve codebase to reintegrate cloud tests ASAP.

[koletzilla]

Summary

This PR adds back cloud tests so it will be executed on PRs again. I feel like they are now in a good enough shape to start executing them also in PRs. It also tests with the new CH Cloud cluster which is more stable than the other one.

Pending things:

• Fix ClickHouse cloud tests #458: This issue should be closed a few days after we merge this PR, but lets wait a bit in case we found other problematic tests.
• Improve test performance #532: We need to also test with other python versions. Testing with other versions is currently not the most problematic thing as we do test them in test_matrix, but it would be nice to also have these tests here. We may add these other python versions once we optimise the tests so they run faster or at least they can run concurrently.

---

Note

Re-enables Cloud testing in CI and makes tests compatible with Cloud limitations.

• CI: Update test_cloud.yml to trigger on PRs to main and set a timeout-minutes: 50 for cloud_smt_tests.
• Tests: Conditionally skip distributed/replicated constraints suites (TestDistributed*ContractColumnsEqual) when DBT_CH_TEST_CLOUD is true, since replicated tables aren’t supported in Cloud; add os import to support skip logic.

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[koletzilla]

Those test were added in #575 but they fail in cloud. The issue is that they test Distributed tables and we cannot test them for cloud. They raise this error:

14:02:16    Compilation Error in model my_model_wrong_order (models/my_model_wrong_order.sql)
  To use distributed materialization cluster setting in dbt profile must be set
  
  > in macro materialization_distributed_table_clickhouse (macros/materializations/distributed_table.sql)
  > called by model my_model_wrong_order (models/my_model_wrong_order.sql)

Skipping them.