[darwin][process]: use a PID above PID_MAX in the not-running tests

Darwin caps PIDs at PID_MAX, which is 99999 (bsd/sys/proc_internal.h), so
99999 is itself assignable and a busy or long-running machine can hand it
out. The test would then look up a live process and quietly stop
exercising the ESRCH path it exists for. 999999 can never be assigned.

TestIOCounters_NonExistent used 999999 until 2114d0d changed it;
TestNumFDs_NonExistent has been on 99999 since it was added in aa5d1fd.