Represents a listener which will be notified about events that occur during a test run.

Example of simple listener, which triggers alarm whenever test fails.

shared class RingingListener() satisfies TestListener {
    shared actual void testError(TestErrorEvent event) => alarm.ring();
}

… such listener can be used directly when creating TestRunner

TestRunner runner = createTestRunner{
    sources = [`module com.acme`];
    extensions = [RingingListener()];};

… or better declaratively with usage of testExtension() annotation

testExtension(`class RingingListener`)
module com.acme;