pytest: don't create 5 nodes in test_fetchinvoice.

CI seems to be OOM killing us; 5 may be too many under valgrind.

VALGRIND=1 pytest tests/test_pay.py::test_fetchinvoice 
Before:
	1 passed in 199.33s (0:03:19)

After:
	1 passed in 177.91s (0:02:57)

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_pay.py

@@ -4499,7 +4499,7 @@ def test_fetchinvoice(node_factory, bitcoind):
                                  'recurrence_label': 'test recurrence'})
 
     # Check we can request invoice without a channel.
-    l4, l5 = node_factory.get_nodes(2, opts={'experimental-offers': None})
+    l4 = node_factory.get_node(options={'experimental-offers': None})
     l4.rpc.connect(l2.info['id'], 'localhost', l2.port)
     # ... even if we can't find ourselves.
     l4.rpc.call('fetchinvoice', {'offer': offer3['bolt12'],
@@ -4510,10 +4510,6 @@ def test_fetchinvoice(node_factory, bitcoind):
     l4.rpc.connect(l3.info['id'], 'localhost', l3.port)
     l4.rpc.call('fetchinvoice', {'offer': offer1['bolt12']})
 
-    # ... even if we are also in gossmap.
-    node_factory.join_nodes([l3, l5], wait_for_announce=True)
-    l4.rpc.call('fetchinvoice', {'offer': offer1['bolt12']})
-
     # Now, test amount in different currency!
     plugin = os.path.join(os.path.dirname(__file__), 'plugins/currencyUSDAUD5000.py')
     l3.rpc.plugin_start(plugin)