test: keep the porter abort case from passing without an abort

[brahyam]

test/porter-sandbox.test.ts's "abort signal kills an in-flight exec" has no
honest outcome on a host without a working setsid.

killableScript wraps the command as exec setsid sh -c …, has the inner
shell record $$ as a process group id, and killScript cancels with
kill -KILL -"$pgid". That is only correct when setsid starts a new session,
which is what makes the inner shell a group leader so pid == pgid.

Where setsid is missing, the wrapper never starts: the exec returns 127
instantly, which satisfies both of the test's assertions — fast, and non-zero.
The case reports green without launching the command, let alone aborting one.

Where a setsid exists that does not create a session (a #!/bin/sh /
exec "$@" shim on PATH is enough), the recorded pid is not a group id, the
group kill matches nothing, and the command runs to completion — failing the
case for a reason unrelated to the code under test.

This probes for the property the mechanism needs, that setsid makes a
command its own process group leader, and skips with that reason when it does
not hold. It then asserts the command started and did not finish, so an exec
that never ran cannot satisfy the case.

Verified in all three states: setsid absent → skips with the reason; a
non-session shim on PATH → skips with the reason; a genuine setsid → runs and
passes in ~0.5s, with the two new assertions holding. Linux CI is the third
state, so coverage there is unchanged apart from the added assertions.

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