pyln-testing: fix catching of memleak/broken errors at exit.

Commit 901342b ("pyln-testing: require
explicit pattern for BROKEN messages.") changed to a manual scan of
logs rather than using is_in_log, so it needs to manually refresh,
otherwise we miss final log messages.

This causes us to often miss memleak messages, which are printed
only in the exit path!

Reported-by: Lagrang3
Fixes: #7565
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Author: rustyrussell

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

@@ -605,6 +605,7 @@ def checkBadGossip(node):
 
 
 def checkBroken(node):
+    node.daemon.logs_catchup()
     broken_lines = [l for l in node.daemon.logs if '**BROKEN**' in l]
     if node.broken_log:
         ex = re.compile(node.broken_log)