test-execute: turn off coredump generation in test services

These services are likely to coredump, and we expect that but aren't
interested in the coredump. Hence let's turn off processing by setting
RLIMIT_CORE to 0/0.

test/test-execute/exec-systemcallfilter-failing.service

@@ -4,6 +4,7 @@ Description=Test for SystemCallFilter
 [Service]
 ExecStart=/bin/sh -c 'echo "This should not be seen"'
 Type=oneshot
+LimitCORE=0
 SystemCallFilter=ioperm
 SystemCallFilter=~ioperm
 SystemCallFilter=ioperm

test/test-execute/exec-systemcallfilter-failing2.service

@@ -4,4 +4,5 @@ Description=Test for SystemCallFilter
 [Service]
 ExecStart=/bin/sh -c 'echo "This should not be seen"'
 Type=oneshot
+LimitCORE=0
 SystemCallFilter=~write open execve exit_group close mmap munmap fstat DONOTEXIST