Refactoring: Use explicit mock expectations for testing push and merge scripts.

- Up to now, mock expectations were simple lists of arguments + return value
- These expectations are now modeled explicitly including the name of the mock (e.g. git or readline)
- The optional test callback function is now explicitly named
- This will allow merging all mock expectation types (e.g. git and readline) into a single list per test case (follow up CL)

TEST=tools/push-to-trunk/script_test.py
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/197313002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19853 ce2b1a6d-e550-0410-aec6-3dcde31c8c00