Thanks Kaniz or your answer. However I think i am struggling to understand some basics. 

Currently i just wrote a simple test function.

def test_something():

assert 1==1

This test passes , but when i make any small change in this function ,it doesnt reflect .

Example i change assert to 1==0, but my output is still same. 

How to refresh main notebook which i am using to run tests files to pick latest change from tests files please? One solution is to restart cluster every time , but that's very bad solution