The only test requires SCYLLA_ENABLE_ERROR_INJECTION. In modes without it
(e.g. release) the suite was empty, so pytest exited with code 5
("no tests collected") and CI failed. Add a no-op case in that branch
so collection always yields at least one test.
Deterministic reproducer using an error injection point placed in
table_helper::insert() between cache_table_info() and execute(). The
test parks fiber A at the injection, drops the target table (evicting
the prepared_statements_cache entry), runs fiber B which nulls
_insert_stmt, then releases fiber A. Without the fix this crashes in
execute(); with the fix fiber A holds a local strong ref and proceeds.
Uses the new waiters() API to synchronize with fiber A's entry into
the injection.
