lightningd: don't return to a default filter level if there are no per-file filters.

In this case, the user's default was info, but they specifically asked for debug
from one plugin.  Since there were no per-file filters, it set filtering to the
default level, info, and rejected it.  Since it's been explicitly filtered in,
we need to pass it at this point.

Reported-by: @wtogami
Fixes: #6503
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_misc.py

@@ -3076,7 +3076,6 @@ def test_log_filter(node_factory):
     assert all([' {}-'.format(l1.info['id']) in l for l in lines])
 
 
-@pytest.mark.xfail(strict=True)
 def test_log_filter_bug(node_factory):
     """Test the log-level option with overriding to a more verbose setting"""
     log_plugin = os.path.join(os.getcwd(), 'tests/plugins/log.py')