Add Test

[azu]

Currently, No test code.

https://github.com/textlint/editor/blob/d6506f6ec8c0663a5e675d97b742401067ce3140/package.json#L32

We need to add some test cases.

• [ ] @textlint/compiler - snapshot testing
  • code generator match snapshot testing
  • https://efcl.info/2018/02/02/snapshot-test/
• [ ] @textlint/config-loader - unit test and snapshot testing
  • It will be moved to textlint core
• [ ] textchecker-element - cypress?
  • We want to detect visual regression?
• [ ] webextension - How?

Probably, we need to add textchecker-element wrapper. Because, textchecker-element example and webextension's contentscript use almost same code base.