Allow OutputCaptureRule to be used directly with AssertJ

[philwebb]

If we make OutputCaptureRule implement CapturedOutput it could be used directly with AssertJ.

[dreis2211]

@philwebb I think there is a chance that #17491 would make OutputCaptureRule obsolete as it wouldn't be used anymore - apart from its own tests. At least the internal one under org.springframework.boot.testsupport.system could be removed. Not sure about the one under org.springframework.boot.test.system as I don't know what your JUnit 4 support plans are, but there should be no internal usages of it anymore as well once #17491 is merged.

Thought I better comment on this - maybe it saves you some work. If not, ignore me and my 0.5x developer thoughts :P

[philwebb]

Thanks @dreis2211. We're going to continue to support JUnit4 externally for quite a while. The internal version is copy/paste at the moment but it'll be a good idea to delete it eventually.