Change RSpec/ExampleWithoutDescription style to single_line_only

Description of the proposal

For empty test description blocks, use specify rather than it do if the test is self-explanatory.

The RSpec/ExampleWithoutDescription cop is enabled by default. Here we are changing the EnforcedStyle of the cop to single_line_only to match the above quoted testing guidelines.

Examples

# bad
it do

# good
it { is_expected.to be_truthy }

# good
specify do

Impact on gitlab-org/gitlab

bundle exec rubocop $f --only RSpec/ExampleWithoutDescription
39218 files inspected, 1181 offenses detected

Check-list

• Mention this proposal in the relevant Slack channels (e.g. #development, #backend, #frontend)
• If there is a choice to make between two potential styles, set up an emoji vote in the MR:
  • CHOICE_A: 🅰
  • CHOICE_B: 🅱
  • Vote yourself for both choices so that people know these are the choices
• The MR doesn't have significant objections, and is getting a majority of 👍 vs 👎 (remember that we don't need to reach a consensus)
• (If applicable) One style is getting a majority of vote (compared to the other choice)
• (If applicable) Update the MR with the chosen style
• Follow the review process as usual

/cc @gitlab-org/maintainers/rails-backend