I could not find any test runner for such concurrency tests. I found only paratest, which can run independent tests, like unit tests parallel.
According to PHP - parallel task runner the best option I think is using pthreads with debug_backtrace. I think it will be hard even with that. I am looking forward the installing problems, thread safety, resource sharing difficulties, backtrace bugs, etc... I will have a great time I am sure...:S
I found async calls in the pthreads examples.
If I ever manage to solve this, I will share it on github and add a link here. Until then...
update
I just realized that I don't need multi thread or multi process applications to test concurrency. For example I can start two transactions with 2 database connections from the same php file. What I need is add event triggering for the statements the db driver does, so I can add breakpoints and wait the other task wherever I want. File locking is just the same... So coroutines or some hand made multi tasking and statement logging is just enough...