Only check for cores at root level

The previous core check would pick up any file in /tmp/ that would start
with the `core` prefix. This included some npm generated files which are
created if MaxCtrl is built on the MaxScale machine.

maxscale-system-test/testconnections.cpp

@@ -769,7 +769,7 @@ int TestConnections::copy_maxscale_logs(double timestamp)
             maxscales->ssh_node_f(i, true, "chmod a+r -R %s", log_dir_i);
         }
 
-        const char* command = "for i in `find /tmp/ -name 'core*'`; do test -e $i && exit 42; done";
+        const char* command = "ls /tmp/core* && exit 42";
         int rc = maxscales->ssh_node_f(i, true, command);
         assert(rc != 42, "Test should not generate core files");
     }