How to test airflow dag in unittest?

I am trying to test a dag with more than one task in the test environment. I was able to test single task associated with the dag but I want to create several tasks in dag and kick of the first task. For testing one task in a dag I am using

task1.run()

which is getting executed. But, the same is not working when I have many tasks one after another in downstream of a dag.

from airflow import DAG
from airflow.operators.bash_operator import BashOperator
from datetime import datetime, timedelta


default_args = {
'owner': 'airflow',
'depends_on_past': False,
'start_date': datetime(2015, 6, 1),
'email': ['airflow@example.com'],
'email_on_failure': False,
'email_on_retry': False,
'retries': 1,
'retry_delay': timedelta(minutes=5),
# 'queue': 'bash_queue',
# 'pool': 'backfill',
# 'priority_weight': 10,
# 'end_date': datetime(2016, 1, 1),
}

dag = DAG('tutorial', default_args=default_args)

# t1, t2 and t3 are examples of tasks created by instantiating operators
t1 = BashOperator(
task_id='print_date',
bash_command='date',
dag=dag)

t2 = BashOperator(
task_id='sleep',
bash_command='sleep 5',
retries=3,
dag=dag)

t2.set_upstream(t1)

t1.run() # It is executing just first task.

In order to run second task I have to run using t2.run() which I don't want as I am designing a DAG. How to achieve this?

from How to test airflow dag in unittest?