Add unit tests for deprecated should_ignore_error method to verify warning and return value

tests/test_should_ignore_error.py

@@ -0,0 +1,7 @@
+import pytest
+from flask import Flask
+
+def test_should_ignore_error_deprecation_warning():
+    app = Flask(__name__)
+    with pytest.warns(DeprecationWarning):
+        assert app.should_ignore_error(None) is False