Chore: Reproduce nested blocks failure (refs #181)

[btmills]

@JounQin I put together a reproduction test tonight to make sure I fully understand the problem, and I'm sharing it here in case it helps as you're working on #183.

[JounQin]

Thanks, I was thinking that has been validated via https://github.com/eslint/eslint-plugin-markdown/pull/183/files#diff-8c591ed3533957b84298776fda1d6d11b2da563b3222e471b4725fe248c0e294

I'll add this test case in that PR.

[btmills]

That fixture was the right .md to reproduce the issue, but by itself it wasn't being executed because no describe()/it() test was using it. If you were to add that fixture by itself without implementing the blocks cache fix yet and run the tests, it wouldn't fail as hoped.

Here I wrote a describe()/it() test that will be executed as part of the test run to prove that 1) preprocess is extracting nested blocks, 2) ESLint is finding the expected lint messages in those nested blocks, and 3) the two layers of postprocess calls map those lint messages back to the correct line in test.md.

The test would function just as well using a file in tests/fixtures/test.md instead of creating the file text with [...].join("\n");, but I personally find it easier to see what the test is doing when the file text and assertions are right next to each other.

[JounQin]

@btmills No worry, I've added this test case at #183 as you suggested.

[btmills]

Closing as this has been integrated into #183, which contains the fix.