T2. add(test): add test API that checks process logs for failures (#3899

)

* Revert "Revert Option<Child> process handling"

This reverts commit 2af3008.

* Add a set of failure regexes to test command output

* Allow debug-printing TestChild again

* When the child is dropped, check any remaining output

* Document a wait_with_output edge case

* Improve failure regex panic output

* Improve builder ergonomics

* Add internal tests for failure regex panics

It would be easy to disable these panics, and never realise.

* Add some module structure TODOs

* Stop panicking if the child process has already been taken

* Add test APIs for consuming child output lines

* Fix a hang on child process drop

* Handle output being already taken in wait_with_output

And document some edge cases we don't handle yet

* Use bash's read command in the TestChild stderr test

And check the actual command we're using to see if it errors.

* Pretty print full failure regex list

* Add the test child command line to the failure regex logs