pytest: fix timing flake in test_invoice_expiry.

Under Postgres, this actually takes more than 2 seconds, so w2
really has timed out already:

```
time.sleep(2)  # total 2
        assert not w1.done()
>       assert not w2.done()
E       assert not True
E        +  where True = done()
E        +    where done = <Future at 0x7fe14e54fee0 state=finished raised RpcError>.done

tests/test_invoices.py:420: AssertionError
```

So space the timeouts out more, and sleep one second too short; the
.result() (which sleeps) will catch up if we were extremely slow.

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

Author: rustyrussell

tests/test_invoices.py

@@ -407,9 +407,10 @@ def test_invoice_expiry(node_factory, executor):
 
     # Test expiration waiting.
     # The second invoice created expires first.
-    l2.rpc.invoice('any', 'inv1', 'description', 10)
-    l2.rpc.invoice('any', 'inv2', 'description', 4)
-    l2.rpc.invoice('any', 'inv3', 'description', 16)
+    # Times should be long enough even for our terrible CI runners!
+    l2.rpc.invoice('any', 'inv1', 'description', 16)
+    l2.rpc.invoice('any', 'inv2', 'description', 10)
+    l2.rpc.invoice('any', 'inv3', 'description', 22)
 
     # Check waitinvoice correctly waits
     w1 = executor.submit(l2.rpc.waitinvoice, 'inv1')
@@ -419,19 +420,18 @@ def test_invoice_expiry(node_factory, executor):
     assert not w1.done()
     assert not w2.done()
     assert not w3.done()
-    time.sleep(4)  # total 6
+    time.sleep(7)  # total 9
     assert not w1.done()
 
-    with pytest.raises(RpcError):
+    with pytest.raises(RpcError):  # total 10
         w2.result()
     assert not w3.done()
 
-    time.sleep(6)  # total 12
-    with pytest.raises(RpcError):
+    time.sleep(5)  # total 15
+    with pytest.raises(RpcError):  # total 16
         w1.result()
     assert not w3.done()
 
-    time.sleep(8)  # total 20
     with pytest.raises(RpcError):
         w3.result()