Require VoidExpect to operate inside an example block

[corsonknowles]

This PR completes branch coverage for VoidExpect.

This PR copies the inside_example? method and re-uses it here to get more coherent behavior from the VoidExpect cop.

Note that if we were thinking of copying inside_example? a 3rd time, I'd suggest factoring it out into a utility (Sandi Metz' rule of 3).

Note that I remove the return true unless parent guard clause, because we expect the inside_example? guard clause to fully cover that condition.

Split from:

• https://github.com/rubocop/rubocop-rspec/pull/1972

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Before submitting the PR make sure the following are checked:

• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [x] Updated documentation.
• [x] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• [x] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

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