pytest: make test_autocleaninvoice more reliable.

We're about to slow down the invoice rpc (esp. under valgrind), which
breaks the delicate timing of the autocleaninvoice test.

Change that so the autocleaner (and timestamp) starts after the invoices
are added.

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

tests/test_invoices.py

@@ -282,11 +282,10 @@ def test_waitanyinvoice_reversed(node_factory, executor):
 def test_autocleaninvoice(node_factory):
     l1 = node_factory.get_node()
 
-    start_time = time.time()
-    l1.rpc.autocleaninvoice(cycle_seconds=8, expired_by=2)
-
     l1.rpc.invoice(msatoshi=12300, label='inv1', description='description1', expiry=4)
     l1.rpc.invoice(msatoshi=12300, label='inv2', description='description2', expiry=12)
+    l1.rpc.autocleaninvoice(cycle_seconds=8, expired_by=2)
+    start_time = time.time()
 
     # time 0
     # Both should still be there.