features test: Pattern to provide additional files in scenario test

[joshspicer]

tracking: #139

This PR defines a pattern for optionally providing additional files needed at build time for a scenario (eg: Dockerfile, build scripts, etc...).

In a Features test folder (eg: ./test/python/), the contents of a folder with the same name as a given scenario (eg: ./test/python/my-cool-scenario for my-cool-scenario) will be copied into the auto-generated .devcontainer folder used during the build.

For example, given a scenario defined in a scenarios.json:

// ...
	"frowning_with_a_dockerfile": {
		"build": {
			"dockerfile": "Dockerfile"
		},
		"features": {
			"smile": {
				"shouldFrown": true
			}
		}
	}
// ... 

The following file tree will result in a succeeding test

.
├── src
│   └── smile
│       ├── devcontainer-feature.json
│       └── install.sh
└── test
    └── smile
        ├── frowning.sh
        ├── frowning_with_a_dockerfile  <------------ new folder   
        │   └── Dockerfile              <----- gets copied over to generated .devcontainer
        ├── frowning_with_a_dockerfile.sh
        ├── scenarios.json
        ├── smiling.sh
        └── test.sh

[chrmarti]

👍