tests: use a more unlikely PID

Tests on openbsd sometimes fail because the random PID generation
collides with the test PID. Since a pid_t is an int32, choose some large
number as the test PID.

It's possible to get a unused PID, either by running through the PIDs
sequentially or generating a random number:

    get_unused_pid(Drv) ->
        PID = crypto:rand_uniform(16#afffffff, 16#ffffffff),
        case alcove:kill(
tests: use a more unlikely PID

Tests on openbsd sometimes fail because the random PID generation
collides with the test PID. Since a pid_t is an int32, choose some
pseudo-random large number as the test PID.

It's possible that the test can still fail since there is a race condition
between testing the PID and using it.