Improve error message for differing output in RuleTester

Right now the assertion looks like this

assert.equal(fixResult.output, item.output, "Output is incorrect.");

Which results in not very helpful output in test runs:

 FAIL  templates/01_eslint-simple-demo.test.js
  ● no-blockless-if › invalid › if (foo) foo()

    AssertionError: Output is incorrect.

It would be much more helpful if the output were more like:

 FAIL  templates/01_eslint-simple-demo.test.js
  ● no-blockless-if › invalid › if (foo) foo()

    AssertionError: Output is incorrect.

    Expected value to be:
      "if (foo) {
      foo()
    }"
    Received:
      "if (foo) foo()"

This is similar to what Jest’s output is. And perhaps @cpojer can give us a little direction on how to make that kind of output using one of Jest’s modules.

I’m happy to contribute this fix. I expect it to be relatively low-risk + high-impact

[vitorbal]

@kentcdodds Thanks for the update! Great job on that PR 😃

[kentcdodds]

This has been merged so the next version of Jest will support the assert module with much better messaging. This is fantastic 🎉