Creates local log of touched approved files

Note: you need to add the following to your .gitignore

.approval_tests_temp/

ApprovalTests now create a temp log in .approval_tests_temp/.approved_files.log after each run that stores all the .approved. files that were touched.
This enables other tooling to do things such as find abandoned files.

Also, we added :
FileUtils.appendToFile(file, txt)