Reported as a part of rust-lang/rust#19120
The logic of rust-lang/rust@74fb798a20 was
flawed because when a CI tool run the test parallely with other tasks,
they all belong to a single session family and the test may pick up
irrelevant zombie processes before they are reaped by the CI tool
depending on timing.
Also, panic! inside a loop over all children makes the logic simpler.
By not destructing the return values of Command::spawn() until
find_zombies() finishes, I believe we can conduct a slightly stricter
test.
Signed-off-by: NODA, Kai <nodakai@gmail.com>