Avoid corrupting umask permanently in the testdir because of fail2ban/server/server.py::start.

In particular:
* https://bugs.gentoo.org/659010#c11
* https://bugs.gentoo.org/790251#c10
* https://bugs.gentoo.org/907350

But see also the many dupes.
--- a/fail2ban/tests/fail2banclienttestcase.py
+++ b/fail2ban/tests/fail2banclienttestcase.py
@@ -23,6 +23,7 @@ __author__ = "Serg Brester"
 __copyright__ = "Copyright (c) 2014- Serg G. Brester (sebres), 2008- Fail2Ban Contributors"
 __license__ = "GPL"
 
+import atexit
 import fileinput
 import os
 import re
@@ -40,6 +41,14 @@ from ..client.fail2bancmdline import Fail2banCmdLine
 from ..client.fail2banclient import exec_command_line as _exec_client, CSocket, VisualWait
 from ..client.fail2banserver import Fail2banServer, exec_command_line as _exec_server
 from .. import protocol
+
+def current_umask():
+	tmp = os.umask(0o022)
+	os.umask(tmp)
+	return tmp
+
+old_umask = current_umask()
+
 from ..server import server
 from ..server.mytime import MyTime
 from ..server.utils import Utils
@@ -48,6 +57,11 @@ from .utils import LogCaptureTestCase, logSys as DefLogSys, with_tmpdir, shutil,
 
 from ..helpers import getLogger
 
+def restore_umask():
+	os.umask(old_umask)
+
+atexit.register(restore_umask)
+
 # Gets the instance of the logger.
 logSys = getLogger(__name__)