Refactor scap backport tests

• Misc refactoring, especially of dependency chain related stuff.
• Split test_depends_on_non_production_branch() off from test_depends_ons() so they can be tested separately.
• Adjustments to avoid unexpected interactions between test cases.
• Stop passing --yes to scap backport. Instead, take advantage of pexpect to validate all of the expected user interactions.
• write_to_file appends a newline to all messages
• Changed various test messages to make them easier to figure out the source of a test commit.
• Sprinkled in some docstrings