Problem/Motivation
In #3432171: dump() no longer produces output in PHPUnit tests running under PHP 8.3 we fixed dump() no longer producing output in PHPUnit tests running in isolation under PHP 8.3, by changing writing to STDERR instead of STDOUT.
However, the fix is not sufficient as later #3452269: dump() calls in tests are producing a PHPUnit\Framework\Exception reported that dump() in kernel tests leads to test failures.
In this issue for PHPUnit 10+, see if we can adopt a different technique instead of using standard streams.
Proposed resolution
Take clues from HtmlOutputLogger and #3453341: Bootstrap HtmlOutputLogger from phpunit.xml, and introduce a PHPUnit extension that captures the output of dump() during the tests, and prints it cumulatively at the end of the testrunner execution. In this, enhance the experience to also add information about where each dump() call happened.
The bigger hurdle here are processes run in isolation (many unit ones, and all kernel/functional/functionaljavascript ones): there's no easy way to return information from the 'isolated' test back to its parent testrunner. However, using an environment variable and a staging file likewise HtmlOutputLogger, we can achieve that.