Fix coredump_phnum_test in case of kern.compress_user_cores=1

PR:		240783
Approved by:	ngie, lwhsu
MFC after:	1 month
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D21776

tests/sys/kern/coredump_phnum_test.sh

@@ -45,10 +45,12 @@ coredump_phnum_body()
 	ulimit -c '$(ulimit -c)'
 	sysctl kern.coredump=$(sysctl -n kern.coredump)
 	sysctl kern.corefile='$(sysctl -n kern.corefile)'
+	sysctl kern.compress_user_cores='$(sysctl -n kern.compress_user_cores)'
 EOF
 
 	ulimit -c unlimited
 	sysctl kern.coredump=1
+	sysctl kern.compress_user_cores=0
 	sysctl kern.corefile="$(pwd)/coredump_phnum_helper.core"
 
 	atf_check -s signal:sigabrt "$(atf_get_srcdir)/coredump_phnum_helper"