test: Assert log warnings and errors (#8457)

Adds `assert_warn`, `assert_error` and `assert_warns_or_errors`
methods to `TestContext`, that let us assert that
a certain warning or error is logged during the test.
Also asserts test logs should not contain any other
errors or warnings.

Adjusts tests accordingly.

Signed-off-by: Jagoda Ślązak <jslazak@jslazak.com>

src/sql/sql_tests.rs

@@ -99,6 +99,8 @@ async fn test_housekeeping_db_closed() {
             _ => {}
         }
     }
+
+    t.assert_warns_or_errors(&["no SQL connection"]).await;
 }
 
 /// Regression test for a bug where housekeeping deleted drafts since their