SchematicTestRunner: ability to run scheduled tasks

Start with the SchematicTestRunner implementation and the task-scheduling path around context.addTask and runner.engine.executePostTasks(); inspect how RunSchematicTask uses the engine workflow. Done means tests can execute only scheduled tasks with a selected name, or RunSchematicTask works in SchematicTestRunner, with coverage for the supported behavior.

If someone uses the SchematicTestRunner and runs a schematic that schedules specific tasks using the context.addTask method, those tasks cannot be run in the tests.

I figured that there is a method (runner.engine.executePostTasks() that runs all scheduled tasks. This is not really helpful though, since there are situations where we only want to run specific tasks and not all scheduled tasks.

This is because some tasks like RunSchematicTask depend on a engine.workflow that is not set up in the SchematicTestRunner. It would be either nice if the RunSchematicTask could work properly in the tests, or there is a method that runs tasks with a specific name.

Current workaround:

/**
 * Due to the fact that the Angular devkit does not support running scheduled tasks from a
 * schematic that has been launched through the TestRunner, we need to manually find the task
 * executor for the given task name and run all scheduled instances.
 *
 * Note that this means that there can be multiple tasks with the same name. The observable emits
 * only when all tasks finished executing.
 */
export function runPostScheduledTasks(runner: SchematicTestRunner, taskName: string) {

  const host = runner.engine['_host'] as EngineHost<{}, {}>;
  const tasks = runner.engine['_taskSchedulers'] as TaskScheduler[];

  return observableFrom(tasks).pipe(
    concatMap(scheduler => scheduler.finalize()),
    filter(task => task.configuration.name === taskName),
    concatMap(task => {
      return host.createTaskExecutor(task.configuration.name)
        .pipe(concatMap(executor => executor(task.configuration.options, task.context)));
    }),
    // Only emit the last emitted value because there can be multiple tasks with the same name.
    // The observable should only emit a value if all tasks completed.
    last()
  );
}

cc. @jelbourn @hansl