enhance(stitch) Test merge conflicts

[gmac]

Adds tests for merge conflicts. These work well enough, although they're pretty opaque for an extremely common use case like picking the right docstring. It'd be nice if there was some way to provide a friendly facade for this type of setup along the lines of:

combineDescriptions({ delimiter: '\n', ignorePrefix: 'ignore!' });

Obviously, we wouldn't want that to interfere with the ability to still take advantage of these merge handlers for other truly specialized use cases. Any ideas for how something like this could be cleanly packaged?

Either way, these tests hit merge handlers so provide worthwhile coverage!

TODO:

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[yaacovCR]

Thanks @gmac!

[theguild-bot]

The latest changes of this PR are available as alpha in npm: 6.2.2-alpha-65b41f0a.0

Quickly update your package.json by running:

npx match-version @graphql-tools 6.2.2-alpha-65b41f0a.0