Format output so that IntelliJ will display a diff button

[robfletcher]

For assertion functions like isEqualTo it's essential to have diff capabilities. Need to research and implement how to format the rendering of the "actual" value so that IntelliJ will know to offer a diff button.

[robfletcher]

This may require coupling to JUnit's API somehow to support multiple diffs in block assertions

[robfletcher]

I'm wondering if we could have

interface AssertionResult

class AssertionFailed : AssertionError(), AssertionResult 
class AssertionPassed : AssertionResult

and group the results in that way.

[robfletcher]

Probably want to depend on https://github.com/ota4j-team/opentest4j