Document a common assertion failure output format so IDEs can integrate "click to show diff" better

[vlsi]

Currently, IDEs hardcode regular expressions which becomes problematic for cases outside of expected.equals(actual).
For instance, if the user compares file contents, they would rather want file contents printed on the screen with a short diff, and then IDE should add click to see diff link based on the file paths.

At the same time, if the expected or actual texts get big, the test framework might want saving them as file, to prevent displaying too large text to the console.

I incline it is not practical to add more text-based regexps for IDE integration, so I wonder if there can be an actual protocol for IDE integration.

See https://youtrack.jetbrains.com/issue/IDEA-313769/Support-click-to-see-difference-for-file-based-comparisons

For example:

{
  "type": "file_content_equal",
  "expected": {"path": "/tmp/expected.txt"}
  "actual": {"path": "/git/repo/actual.txt", "updateable": "true"}
}

updateable: true could mean the IDE could add a button to update actual file based on the contents of the expected one.