- Implemented the logic and ordering of PR #818. To oversimplify, user configuration is assembled first and then all defaults are applied. A handful of methods have been broken into two renamed methods, one for default handling and another for user configuration handling.
- Added lots of comments and updated `Configurator` method names to use _populate_, _merge_, etc. consistently.
- Refactored `TestRunnerManager` to move configuration validation into Configurator & friends
- Renamed Backtrace `gdb` tool definition to match other tools and added support for the tool argument shortcut in project configuration.