Add TestCase::registerFailureInterface() to register interface that marks exceptions to be treated as failures, not errors

[sebastianbergmann]

Motivated by phpspec/prophecy-phpunit#45 (comment)

[sebastianbergmann]

@stof @Jean85 Any feedback on this prior to the release of PHPUnit 10.1 would be much appreciated.

[Jean85]

I've integrated it into phpspec/prophecy-phpunit#45, thanks!
As of now though, we're still blocked by phpspec/prophecy#585, so it's not tested under CI yet (but locally it seems to work).

As for comments, only two come to mind for now:

• since you're using instanceof, it works with classes too, not only interfaces; which is good! But maybe it warrants a rename?
• I would record FQCNs as keys, not values, to avoid duplications (multiple calls would be idempotent)

[sebastianbergmann]

since you're using instanceof, it works with classes too, not only interfaces; which is good! But maybe it warrants a rename?

Makes sense, implemented in 28766f5.

I would record FQCNs as keys, not values, to avoid duplications (multiple calls would be idempotent)

Makes sense, implemented in a249d88.

Thank you!