Skip tests depending on coredumps if coredumps are disabled via kern.coredump.

The kern.coredump sysctl can be set to 0 to disable coredumps. Skip the
'status_coredump' and 'wait6_coredumped' tests if this sysctl is set to 0
rather than reporting a failure.

Submitted by: brooks
Reviewed by: ngie
Obtained from: CheriBSD
Sponsored by: DARPA / AFRL
Differential Revision: https://reviews.freebsd.org/D10665