Check tasks should not provide same values

Check is added to linear_flow and unordered_flow patterns that
there are no two child items provide same values.

Unit tests added.

Change-Id: I7a293d58a962de37ba2fd1dd989bf0a218417430
Closes-Bug: #1264995
Enable H202 warning for flake8

Change-Id: Iaeb6d4ab1af5063558ef1a7b9a85e46c6cf0ccb7
Merge "Enable H202 warning for flake8"