Refactor TestThreadInstrumentation to investigate CI flakiness

`test_thread_instrumentation_fork_safe` has been failing occasionaly
on Ubuntu and Arch. At this stage we're not sure why, all we know is
that the child exit with status 1.

I suspect that something entirely unrelated cause the forked children
to fail on exit, so by using `exit!(0)` and doing assertions in the
parent I hope to be resilient to that.

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