tests: fix bounce test paths to be __file__-relative

Previously used CWD-relative paths which only worked when testall.py
was invoked from the prefix root (e.g. python3 tests/testall.py).
Using __file__-relative paths works regardless of invocation directory.

Author: thegushi

tests/test_bounces.py

@@ -21,6 +21,8 @@
 import os
 import unittest
 import email
+
+_TESTDIR = os.path.dirname(os.path.abspath(__file__))
 try:
     from Mailman import __init__
 except ImportError:
@@ -192,7 +194,7 @@ def test_bounce(self):
         for modname, file, addrs in self.DATA:
             module = 'Mailman.Bouncers.' + modname
             __import__(module)
-            fp = open(os.path.join('bounces', file))
+            fp = open(os.path.join(_TESTDIR, 'bounces', file))
             try:
                 msg = email.message_from_file(fp)
             finally:
@@ -212,7 +214,7 @@ def test_bounce(self):
     def test_SMTP32_failure(self):
         from Mailman.Bouncers import SMTP32
         # This file has no X-Mailer: header
-        fp = open(os.path.join('bounces', 'postfix_01.txt'))
+        fp = open(os.path.join(_TESTDIR, 'bounces', 'postfix_01.txt'))
         try:
             msg = email.message_from_file(fp)
         finally: