test: Fix FATAL: sorry, too many clients already

Within `BasePostgresSDKTests`, new database connections via SQLAlchemy
haven't been closed, and started filling up the connection pool,
eventually saturating it.

Author: amotl

target_postgres/tests/test_sdk.py

@@ -61,7 +61,8 @@ class BasePostgresSDKTests:
     @pytest.fixture()
     def connection(self, runner):
         engine = create_engine(runner)
-        return engine.connect()
+        with engine.connect() as connection:
+            yield connection
 
 
 SDKTests = get_target_test_class(