Add coverage analysis to push and merge script tests.

The coverage analysis must live in an extra file in order to analyze the scripts when being imported.

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

Review URL: https://codereview.chromium.org/183923013

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19640 ce2b1a6d-e550-0410-aec6-3dcde31c8c00