Fix GH-18404: Display diff when a single test is run

[alexandre-daubois]

Fix #18404

[iluuu1994]

We use this for development quite frequently, and prompting will get quite annoying. The original aim of this file was to be shared with php-src devs, but when you're running a subset of tests I'm not sure this applies any longer? At that point, it seems like you're investigating the issue on your own.

Another option might be to enable the --show-diff option by default, or at least when specifying a subset of tests to run.

[alexandre-daubois]

Hmm, I see. I'm afraid displaying the diff for a subset would be too much as well, the summary of failed test is useful. Maybe we could display the diff if only one test is ran and fails? I think this use case would really help having a faster debugging workflow

[iluuu1994]

I'd be ok with this approach. --enable-diff is always enabled in my environment, I don't find it particularly noisy. run-tests.php is very noisy anyway. Somewhat related: #16443

[alexandre-daubois]

Updated accordingly: diff is displayed when one only one test is run and fails.