pytest: Exclude elements-cli from valgrind when following children

Without this we'd be attaching valgrind to all elements-cli call, resulting in
a huge overhead and timeouts.

tests/utils.py

@@ -959,7 +959,7 @@ class NodeFactory(object):
                 'valgrind',
                 '-q',
                 '--trace-children=yes',
-                '--trace-children-skip=*python*,*bitcoin-cli*',
+                '--trace-children-skip=*python*,*bitcoin-cli*,*elements-cli*',
                 '--error-exitcode=7',
                 '--log-file={}/valgrind-errors.%p'.format(node.daemon.lightning_dir)
             ]