test_lightningd: fix valgrind detection on children.

I tracked down a bug, and couldn't figure out why valgrind wasn't
finding it.

From man valgrind:

       --log-file=<filename>
           Specifies that Valgrind should send all of its messages to the
           specified file. If the file name is empty, it causes an abort.
           There are three special format specifiers that can be used in the
           file name.

           %p is replaced with the current process ID. This is very useful for
           program that invoke multiple processes. WARNING: If you use
           --trace-children=yes and your program invokes multiple processes OR
           your program forks without calling exec afterwards, and you don't
           use this specifier (or the %q specifier below), the Valgrind output
           from all those processes will go into one file, possibly jumbled
           up, and possibly incomplete.

"possibly incomplete" indeed!

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

tests/test_lightningd.py

@@ -9,6 +9,7 @@ import json
 import logging
 import os
 import random
+import re
 import sqlite3
 import string
 import sys
@@ -114,7 +115,7 @@ class NodeFactory(object):
                 '--trace-children=yes',
                 '--trace-children-skip=*bitcoin-cli*',
                 '--error-exitcode=7',
-                '--log-file={}/valgrind-errors'.format(node.daemon.lightning_dir)
+                '--log-file={}/valgrind-errors.%p'.format(node.daemon.lightning_dir)
             ] + node.daemon.cmd_line
 
         node.daemon.start()
@@ -135,10 +136,14 @@ class BaseLightningDTests(unittest.TestCase):
         self.node_factory = NodeFactory(self, self.executor)
 
     def getValgrindErrors(self, node):
-        error_file = '{}valgrind-errors'.format(node.daemon.lightning_dir)
-        with open(error_file, 'r') as f:
-            errors = f.read().strip()
-        return errors, error_file
+        for error_file in os.listdir(node.daemon.lightning_dir):
+            if not re.match("valgrind-errors.\d+", error_file):
+                continue;
+            with open(os.path.join(node.daemon.lightning_dir, error_file), 'r') as f:
+                errors = f.read().strip()
+                if errors:
+                    return errors, error_file
+        return None, None
 
     def printValgrindErrors(self, node):
         errors, fname = self.getValgrindErrors(node)