Rework snapshot matcher to support parallel mocha

[bajtos]

Extracted from #5011 to land the changes in isolation from the actual enablement of parallel testing.

The first commit is cherry-picked 20da28e, then I added more commits to clean up the code more to my liking.

My intention is to further clean up the commit history once we agree on the implementation direction & details.

To verify parallel tests, one can run the test suite via npm test -- --parallel. Some of the tests are failing in parallel mode on a timeout, I think that's something to investigate as part of enabling parallel mode (outside of this PR).

Checklist

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• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated

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[raymondfeng]

@bajtos Please fix code formatting.

[raymondfeng]

lodash is a much smaller dependency than other ones we already have for @loopback/build, such as typescript, nyc, mocha, prettier.

[bajtos]

lodash is a much smaller dependency than other ones we already have for @loopback/build, such as typescript, nyc, mocha, prettier.

Fair enough.

Can we use https://lodash.com/docs/4.17.15#mergeWith instead?

I am not comfortable with using mergeWith's deep merge algorithm, I am concerned that non-scalar config options (like require) may require specific handling and the default deep-merge implementation may create unwanted outcomes.

Let's use assignWith for the first version, learn more about real-world usage and then improve the merging algorithm as needed.

[bajtos]

I rebased the patch on top of the latest master, addressed review comments and made few more improvements/cleanups:

• fix linter issues

• simplify mergeMochaConfigs using lodash

• clean up packages/cli/.mocharc.js

• improve comment in root .mocharc.js

• cleanup snapshot matcher:

  1. Rename "state" to "snapshotMatcher" ("matcher"). I find "state" as too ambiguous, let's think about the new design as having one SnapshotMatcher instance for each snapshot directory. It is possible to further refactor the code from current functional-programming style into OOP by introducing SnapshotMatcher class, but I feel such change is not needed now.
  2. Move definition of root hook functions out of the hooks object, to make it easier to build a high-level understanding of the hooks without having to skip through implementation details.
  3. Move "currentTest" from individual states (matcher) into a single global variable, there is no need to duplicate it in every snapshot matcher instance.

@raymondfeng I consider the PR as ready for final review, PTAL.

[bajtos]

(I'll clean up the commit history before merging.)