Fix false positive alerts from a run-pass test on Command.

Reported as a part of rust-lang/rust#19120

The logic of rust-lang/rust@74fb798a200dc82cf5b4a18065e3ea565229adc3 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.