test: PR_DEATHSIG is reset after changing id/gid

test: PR_DEATHSIG is reset after changing id/gid

Child processes are left hanging on abort() as these child procs
freeze(), so test suites hang as well when test-namespace fails,
and processes are leaked.

From the docs:

The parent-death signal setting is also cleared upon changes to any of
the following thread credentials: effective user ID, effective group ID,
filesystem user ID, or filesystem group ID.

Set the deathsig again after changing id.

Follow-up for 2ade8218598afba0802b1007535b5c8deaeceb58