Linear flow: mark links and rework unit tests

- add 'invariant': True to all links produced by linear flow;
- write new unit tests for basic functionality of linear flow
  pattern;
- remove some simple now-redundant unit tests from
  test_flow_dependencies.

Change-Id: I1f4fa8a1dfc61485555a10e8d0629a67aab1997f
Merge "Linear flow: mark links and rework unit tests"