Provide machine readable interface for burn_in_tests

burn_in_tests provides a way to discover and run the tests/suites that have changes locally or in a given patch build. When running in a patch build, we use 'generate.tasks' to run the tests in separate tasks. It would be nice to move the generate tasks logic to mongo-task-generator so that the logic required can be consolidated to a single place.

The first step in order to do this would be to provide an interface to burn_in_tests that would provide a what tests/tasks should be run in a machine readable way. A straight-forward way of doing this would be to add a `--yaml` flag to the burn_in_tests script that outputs the tests to tasks map in yaml format.

As a database contributor,
I want to be able to get burn_in_tests discovery in yaml format,
So that other tools can easily query and use the information discovered by burn_in_tests.

AC:

• There is a way to run burn_in_tests in a mode that will output its discovery in an easily parsable format.