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

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

Committed: https://code.google.com/p/v8/source/detail?r=19853

[Michael Achenbach, 2014-03-12 13:28:33]

PTAL

[Jarin, 2014-03-12 14:34:30]

lgtm

[Michael Achenbach, 2014-03-12 14:39:15]

Committed patchset #1 manually as r19853 (presubmit successful).