Cleanup all sqlalchemy sessions before each test (#33190)

Attempt to solve some of the flakiness we saw recently - this
change will run closing all opened sqlalchemy sessions before each
test. This should be a little slower than before - especially
for the very fast tests, but it provide us with much better
isolation between the tests - thus avoiding the flakiness
that we observe recently - as documented in #33178

Hopefully this one

Fixes: #33178

tests/conftest.py

@@ -942,3 +942,12 @@ def initialize_providers_manager():
     from airflow.providers_manager import ProvidersManager
 
     ProvidersManager().initialize_providers_configuration()
+
+
+@pytest.fixture(autouse=True, scope="function")
+def close_all_sqlalchemy_sessions():
+    from sqlalchemy.orm import close_all_sessions
+
+    close_all_sessions()
+    yield
+    close_all_sessions()