pdeathsig_helper and .debug/pdeathsig_helper.debug placed in /mnt when DESTDIR=/mnt

[Trond Endrestøl]

I noticed two new entries in / after running make installworld today 
using amd64 stable/11 r333390:

# LANG=en_US.UTF-8 ls -lT /pdeathsig_helper /.debug/pdeathsig_helper.debug
-r--r--r--  1 root  wheel  7528 May  9 12:06:58 2018 /.debug/pdeathsig_helper.debug
-r-xr-xr-x  1 root  wheel  8576 May  9 12:06:58 2018 /pdeathsig_helper

I'm sure these belong in /usr/tests/sys/kern. This must have happened 
in r333162 or shortly after.

See PR 228018, of which I hijacked.

-- 
Trond.