This very simple page allows you to publish an assetlinks.json file at the location [domain.com]/.well-known/assetlinks.json, for the purpose of testing.
To make sure this works, you must:
Use the name [yourgithubusername].github.io for the repository name. Put the following text at the beginning of your assetlinks.json file:
Yes, this is invalid JSON, but the actual served content will be fine.