Accept pending cops in the development build

[koic]

Follow up of #7646 (comment).

This PR accepts pending cops in the development build.

Some tests fail with the following warnings when using the Enabled: pending status.

% bundle exec rake spec
(snip)

1) RuboCop::CLI --only when one cop is given accepts cop names from
plugins
     Failure/Error: raise output

     RuntimeError:
       The following cops were added to RuboCop, but are not
configured. Please set Enabled to either `true` or `false` in your
`.rubocop.yml` file:
        - Style/ZeroLengthPredicate
       Inspecting 2 files
       ..

       2 files inspected, no offenses detected
     # ./spec/rubocop/cli/cli_options_spec.rb:213:in `block (4 levels)
in <top (required)>'
     # ./spec/support/cli_spec_behavior.rb:26:in `block (2 levels) in
<top (required)>'
     # ./lib/rubocop/rspec/shared_contexts.rb:29:in `block (4 levels) in
<top (required)>'
     # ./lib/rubocop/path_util.rb:67:in `chdir'
     # ./lib/rubocop/path_util.rb:67:in `chdir'
     # ./lib/rubocop/rspec/shared_contexts.rb:28:in `block (3 levels) in
<top (required)>'
     # ./lib/rubocop/rspec/shared_contexts.rb:8:in `block (2 levels) in
<top (required)>'

This PR prevents those tests from catching a pending cop warning.
OTOH, this PR supplements the E2E test for a pending cop warning.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

Thanks!