Don't dump core files with lib/libc/ssp/ssp_test and lib/libc/gen/assert_test

The default sysctl kern.corefile value is compatible with kyua test (FreeBSD
will dump to the current directory). If it's set to an absolute path however,
kyua test will not be able to clean up the corefiles after the fact

The corefiles have little value when testing the behavior of feature behavior,
so just disable corefile generation

MFC after: 1 week
Obtained from: Isilon OneFS (^/onefs/head@r511419)
Sponsored by: EMC / Isilon Storage Division