Epic: Faster Behat test runs #394

The Behat suites are the bulk of the CI time for the WP-CLI packages. A pull request against entity-command runs 51 Behat jobs that add up to about 15 hours of runner time, and individual jobs take between 7 and 45 minutes. Locally, a full run of a single package can take tens of minutes.

This epic collects the changes that should make these runs faster without weakening what the tests verify.

Every change follows the same approach:

1. Use the CI baseline recorded in Measure Behat run times reliably #384.
2. Open a draft PR with the change.
3. Run the same CI jobs against it.
4. Record both measurements on the issue, so each change is decided on actual numbers.