imgdiag: Fix rarely flaky imgdiag_test.

Don't hardcode pid as we could get unlucky and a real process
could have a pid of 12345.

This prevents false negatives by making the PID always be out-of-range
for valid kernel PIDs.

Change-Id: I8e8ae5c20a39e95b27b35905d5273c77dbf701f9
1 file changed