pytest: add slow_test marker.

And when it's set, and we're SLOW_MACHINE, simply disable valgrind.

Since Travis (SLOW_MACHINE=1) only does VALGRIND=1 DEVELOPER=1 tests,
and VALGRIND=0 DEVELOPER=0 tests, it was missing tests which needed
DEVELOPER and !VALGRIND.

Instead, this demotes them to non-valgrind tests for SLOW_MACHINEs.

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

contrib/pyln-testing/pyln/testing/fixtures.py

@@ -174,6 +174,7 @@ def teardown_checks(request):
 @pytest.fixture
 def node_factory(request, directory, test_name, bitcoind, executor, db_provider, teardown_checks, node_cls):
     nf = NodeFactory(
+        request,
         test_name,
         bitcoind,
         executor,