[BUG] Add more tests coverage for AD task runner and task manager

[Zhangxunmt]

The jacocoExclusions list includes two important classes in the AD tasks layer. https://github.com/opensearch-project/anomaly-detection/blob/main/build.gradle#L547. They lack tests coverage which may cause bugs. We should add more tests to meet the bar for the following classes.

    // TODO: fix unstable code coverage caused by null NodeClient issue
    // https://github.com/opensearch-project/anomaly-detection/issues/241
    'org.opensearch.ad.task.ADBatchTaskRunner',
    'org.opensearch.ad.task.ADTaskManager',

    'org.opensearch.ad.AnomalyDetectorRunner'
    // TODO: unified flow caused coverage drop
    'org.opensearch.ad.transport.DeleteAnomalyResultsTransportAction',
    //TODO: custom result index caused coverage drop
    'org.opensearch.ad.transport.handler.AnomalyResultBulkIndexHandler'