Refactor system tests

[jeddy3]

Which issue, if any, is this issue related to?

This lays the foundation for being able to test bundled code against real-world examples, closing #2454 and #3935.

Is there anything in the PR that needs further explanation?

The system tests were designed to:

... assert that we end up with some expected output, given a configuration and a stylesheet. They should reproduce real use-cases and verify that those use-cases work as expected.

Somewhere along the line, we muddied the waters. Tests that were not related to real-world cases, didn't use real-world configs, nor snapshotted the results were added. The tests often duplicated specific fixtures from the __tests__/fixtures directory into the config.json and stylesheet.css files of the system tests.

This has led to a fragmentation of the tests.

Additionally, the landscape has changed. The original system tests were added before:

• the autofix feature existed
• the Bootstrap and Sanitize.css teams used stylelint
• our refactoring to produce a browser bundle

I've used this opportunity to update the system tests to reflect this.

In this pull request, I've:

• moved the non-system tests into lib/__tests__, making use of existing fixtures or creating new ones where appropriate
• moved replaceBackSlashes into testUtils, cleaning up the __tests__ directory
• removed the duplicated replaceBackSlashes function from systemTestUtils.js
• updated the system tests to:
  • test three real-world examples:
    • a Bootstrap SCSS source file and their config
    • the Sanitize.css source file and their config
    • the Zen Garden CSS and the standard config (as an example of using running stylelint to fix an old code base)
  • support snapshotting fixed code
  • support using the code and config properties, rather than file and configFile ones, to avoid filesystem access

[hudochenkov]

I believe it's possible to do in Node.js 10+:

const { copyFile } = require('fs').promises;

or

const { copyFile } = require('fs/promises');

https://nodejs.org/api/fs.html#fs_fs_promises_api
https://nodejs.org/api/fs.html#fs_fspromises_copyfile_src_dest_mode

[jeddy3]

That's odd. I originally wrote it using the promise API but refactored it after the node/no-unsupported-features ESLint plugin warned against using it. I'll take another look because it appears to be a stable API in node@10.

[hudochenkov]

Linter plugin false positive mysticatea/eslint-plugin-node#174

[jeddy3]

I'll use the API and disable the ESLint warning for now.

[hudochenkov]

Same as comment above. promisify is not needed here.

[hudochenkov]

For safety let's use different names for rest, so we're not confused in results scope what rest we're dealing with.

Also we can use single way of removing unneeded property:

		results: results.map(({ _postcssResult, source, ...rest }) => {
			return rest;
		}),

Or:

		results: results.map((result) => {
			delete result.source;
			delete result._postcssResult;

			return result;
		}),

[jeddy3]

Changes made.

[hudochenkov]

Good work!

[m-allanson]

Good stuff.