Add eslint-plugin-mocha to eslint configuration.

[jodator]

Suggested merge commit message (convention)

Feature: Add "eslint-plugin-mocha" rules to the default ESLint configuration and configure dangling-coma rule. Closes #481. Closes ckeditor/ckeditor5#6246.

---

Additional information

• Full list of rules: https://github.com/lo1tuma/eslint-plugin-mocha/tree/master/docs/rules
• I did a quick check of a current code base and there are some minor issues to solve with proposed configuration:

1. These rule invalidates test() to be used inside it() as it prevents nesting it() or test() - which is an error. (simple rename test() to something else will do)
2. Some beforeEach() duplication on the same level
3. Some false-positives for no-top-level-hooks (this rule important to keep)

PRs for those will be linked here.

[jodator]

I've configured below rules for Mocha:

level	fixable	rule	description
error		handle-done-callback	enforces handling of callbacks for async tests
error	🔧	no-async-describe	disallow async functions passed to describe
error		no-exclusive-tests	disallow exclusive mocha tests
error		no-global-tests	disallow global tests
error		no-nested-tests	disallow tests to be nested within other tests
error		no-pending-tests	disallow pending/unimplemented mocha tests
error		no-sibling-hooks	disallow duplicate uses of a hook at the same level inside a describe
error		no-top-level-hooks	disallow top-level hooks
warn		no-identical-title	disallow identical titles

and dangling-comma.

[coveralls]

Coverage remained the same at 91.604% when pulling fd239d9 on i/481 into 150fd8e on master.

[jodator]

ps.: I've found only this much .only removals (the bolded ones are mine :P )