Stabilize `ensure_execute_processes_have_correct_num_threads` test (#7253)

Saw this test flake a few times, last time
[here](https://github.com/paritytech/polkadot-sdk/actions/runs/12834432188/job/35791830215).

We first fetch all processes in the test, then query `/proc/<pid>/stat`
for every one of them. When the file was not found, we would error. Now
we tolerate not finding this file. Ran 200 times locally without error,
before would fail a few times, probably depending on process fluctuation
(which I expect to be high on CI runners).