Make a manually-run test to generate a saved document with full feature coverage

[Keavon]

Right now, the check_if_graphite_file_version_upgrade_is_needed test opens the graphite-test-document.graphite file and checks if the deserialization fails due to an outdated format. If it fails, the developer should bump the document version number and manually save a new file to replace it.

But that file doesn't use all the features (such as layer types) that are serialized out of the set of all possibilities. The developer should try to make this use lots of features. But this should really be a manually-run test (which is disabled in the test suite, but can be run by the developer) which has the side effect of replacing the file with one that uses the editor and makes some layers of different types in order to maximize coverage of the features used and serialized into the file.

There should be instructions on how to run the test, and a reminder to bump the GRAPHITE_DOCUMENT_VERSION.