Add initial tests for strategies and searchers

Some basic tests to determine if the strategies and searchers can
correctly locate the expected commits. Add helper functions to the
base class to be able to provide a simple structure of node
relationships for a git repository to be built.

Include a more complex test for both the default walker strategy and
upstream merge base commit searcher that is currently expected to
fail.

Change-Id: I19db2de82c5a3d661fececbea8b23ddb33400df0
Merge "Add initial tests for strategies and searchers"