Fix flaky test tests/test_account_sharing.py::test_queries_stay_constant_whatever_the_accounts_and_nodes

The merge lane saw `tests/test_account_sharing.py::test_queries_stay_constant_whatever_the_accounts_and_nodes` fail and then pass on a rerun of the same tree. - Test: `tests/test_account_sharing.py::test_queries_stay_constant_whatever_the_accounts_and_nodes` - Tree: `76b67aad8a42cb5f5be3bcfa83411c75cffed923` - Lane task: 9940 - Flake record: 01a1743d-67d3-70fa-b293-0de257c524cc Make it deterministic; do not delete it unless it tests nothing.

Deflaked the constant-queries test: the statement counter now counts only the test thread's SQL (it was counting background threads too). Old test fails 26≠8 under background SQL; new one passes. Quick gate green; PR #6886 queued in the merge lane.