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.
1. It will do no harm anyways.
2. This test code doesn't assume what will be returned in the `Err` case.  Then running `find_zombies()` before destruction of the `Err` case values means a (very slightly) stricter test.