protected DataSubscriber<Object> verifyGetAndSubscribe(
        Supplier<DataSource<Object>> dataSourceSupplier, DataSource<Object> underlyingDataSource) {
      return verifyGetAndSubscribe(dataSourceSupplier, underlyingDataSource, false);
    }
Verifies that our mDataSourceSupplier got underlying data source and subscribed to it.
Subscriber is returned.
fbcore/src/test/java/com/facebook/datasource/DataSourceTestUtils.java