Drop RSpec/EmptyExampleGroup customization

[pirj]

We're providing a flexible way of customizing RSpec syntax thanks to #956

This is not based on #956, but rather includes it (can't specify a base branch from another fork).
Those are the changes 42f7708
This is the change needed to avoid mutating the default config between the examples 718b398

This way it's possible to drop customization of this cop instead customize via .rubocop.yml configuration, i.e.:

AllCops:
  RSpec:
    Language:
      Includes:
        Example:
        - it_has_special_behavior

There's a problem with specs, though, specifically with this:

RuboCop::ConfigLoader.default_configuration.for_all_cops.dig('RSpec', 'Language')

for_all_cops is memoized in Config:

# rubocop-0.89.1/lib/rubocop/config.rb:132:

def for_all_cops
  @for_all_cops ||= self['AllCops'] || {}
end

It's not so obvious to notice, since in this cop the example that is modifying the language is in its own context, and it runs last.
Even if you add an example after it, due to how RSpec orders examples, it will still be executed last.
However, if you add a context with an example, you'll see that the language changes leaked.

We need to figure out a way to reset the config between examples, and do it in a performant way.
Thoughts? dup in before and assign back in after?

Fixes #969

---

Before submitting the PR make sure the following are checked:

• [no] Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have modified an existing cop's configuration options:

• Set VersionChanged in default/config.yml to the next major version.

[sl4vr]

This might work

[pirj]

Found a way around. Please take a look.

[pirj]

I'll rebase this one.

[pirj]

I'll rebase this one.

Nevermind, #956 comes first.

[pirj]

It's time. I'll rebase this one.

PS Once #956 is merged 🤦

[pirj]

Rebased on top of the current #956

[pirj]

JRuby crashed on rake internal_investigation, which is quite interesting. I guess I'll have to add :config to all Concept specs.

[pirj]

I can reliably reproduce the issue with --seed 37363:

Failure/Error: Language.config['HookScopes'].include?(element.to_s)
undefined method `[]' for nil:NilClass
./lib/rubocop/rspec/language.rb:127:in `all'

[pirj]

There seems to be something broken in the edge RuboCop, I'm looking into it.

[bquorning]

There seems to be something broken in the edge RuboCop

I this is the change we’re looking for: rubocop/rubocop@51ff1d7#diff-0b9b1074946b724df02392b4d1fac4ccbc9ecff5c71166921d20c41a72ec1c14

If you want to merge, we can fix compatibility with edge in a later PR.

[pirj]

The failure on edge-rubocop is irrelevant. Notified RuboCop guys about a breaking API change here rubocop/rubocop@51ff1d7#r43837211