How do you write MUnit tests and what are the key components?

MUnit is MuleSoft's testing framework. Key components:

1. MUnit Test: A test case with an optional Before/After Test or Before/After Suite section.
2. Mock When: Intercepts a specific component (by name) and returns a mocked payload/attributes instead of executing it. Prevents actual HTTP calls or database queries during tests.
3. Set Event: Sets the payload and attributes for the test input.
4. Verify Call: Asserts that a specific processor was called (or not called) with specific parameters.
5. Assert That: Assertions using DataWeave matchers — equalTo(value), notNullValue(), hasSize(n), containsString('text').
6. Spy: Observe a component's behaviour without mocking it. Tests should cover happy path, error paths, and boundary conditions. MUnit runs as part of Maven lifecycle (mvn test) and integrates with CI/CD pipelines.