Fix proc test flakiness
Fix proc test flakiness

Thread from earlier test can show up in `/proc/self/tasks` while the
thread tears down. Account for that when searching for procs for the
first time in the test.

PiperOrigin-RevId: 361689673

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor