pytest: fix feature mask for EXPERIMENTAL_FEATURES, add wumbo support.

There are various places where our tests failed with
--enable-expimental-features.  And our plugin test overlapped an
existing feature.

We make our expected_feature functions more generic, and use them
everywhere.

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

tests/test_misc.py

@@ -1880,6 +1880,8 @@ def test_list_features_only(node_factory):
                 'option_payment_secret/odd',
                 'option_basic_mpp/odd',
                 ]
+    if EXPERIMENTAL_FEATURES:
+        expected += ['option_unknown_102/odd']
     assert features == expected