THREAD_{BEGIN,END} events include null fields
Fix indentation
Simplify print_backtrace method
Remove unnecessary require
Simplify debug_proc test method
Clean up test_helper a bit
Extract an utility method
Tweak debug_code method to load a real file.

The new version is as resilient as the previous one, but has the
advantage that we can make assertions on both the file name and line
numbers now.
Standarize a couple of comments
Migrate frame tests to use debug_code method
Fix bug: set fullpath was not showing fullpaths