`start_xvfb` treated `--display-number 0` as omitted because it used the
option's truthiness, so an explicit `:0` was replaced by a random display
and its collision check ran against that random display instead.
Select the random display only when the option is absent, and add a
fixture-backed regression test covering explicit zero, an occupied
explicit zero, explicit one, and omission.
Fixes#5189
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
