Unit tests for unordered flow

This change adds unit tests that check unordered flow methods
without loading it into engine and executing it.

Change-Id: I665b6fe070759061edf7a7da4f117c0269c9c3cf
Rewrite assertion for same elements in sequences

New version is able to compare sequences whith elements that are not
hashable and cannot be compared (so that sorted() does not work).
For that, a utility function that caluclates difference between two
sequences was added. This function was also used in retry.ForEachBase.

The assertion was also renamed to assertItemsEqual, which matches
same assertion that was added in python 2.7.

Change-Id: I2b1b811190e9dc51718e4ca17ffc5c9015c34dc4
Rework graph flow unit tests

This commit adds unit tests that check graph flow methods without
executing or flattening it. Now-redundant tests from other test
suites are deleted.

Change-Id: I8dafe0f9b295428831eddb3f9fd48f042d2f1ffc
Merge "Unit tests for unordered flow"
Merge "Rewrite assertion for same elements in sequences"
Merge "Rework graph flow unit tests"