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/tools/tools_tests.rs

@@ -328,6 +328,7 @@ async fn test_file_handling() {
 
     assert!(delete_file(context, Path::new(fn0)).await.is_ok());
     assert!(!file_exist!(context, &fn0));
+    t.assert_warn("refusing to delete non-file").await;
 }
 
 #[test]