Use a short tempdir for UNIX-domain sockets in t_changepw.py.

Otherwise, we get a UNIX domain socket path too long error.
Mirror the approach already used by t_ccache.py / t_otp.py

--- a/tests/t_changepw.py
+++ b/tests/t_changepw.py
@@ -1,10 +1,14 @@
 from k5test import *
+import tempfile

-# Also listen on a UNIX domain sockets for kpasswd.
+# sun_path is limited to 108 bytes on Linux; the portage build path
+# can push $testdir/*.sock past that limit. Use a short tempdir
+# instead, matching what t_ccache.py / t_otp.py already do.
+sockdir = tempfile.mkdtemp()
 unix_conf = {'realms': {'$realm': {
-    'kdc_listen': '$port0, $testdir/krb5.sock',
-    'kadmind_listen': '$port1, $testdir/kadmin.sock',
-    'kpasswd_listen': '$port2, $testdir/kpasswd.sock'}}}
+    'kdc_listen': '$port0, %s/k' % sockdir,
+    'kadmind_listen': '$port1, %s/a' % sockdir,
+    'kpasswd_listen': '$port2, %s/p' % sockdir}}}
 realm = K5Realm(create_host=False,get_creds=False, kdc_conf=unix_conf)
 realm.start_kadmind()
 realm.prep_kadmin()
@@ -56,9 +60,9 @@ realm.run([kadminl, 'delprinc', 'testprinc'])
 mark('password change over UNIX domain socket')

 unix_cli_conf = {'realms': {'$realm': {
-    'kdc': '$testdir/krb5.sock',
-    'admin_server': '$testdir/kadmin.sock',
-    'kpasswd_server': '$testdir/kpasswd.sock'}}}
+    'kdc': '%s/k' % sockdir,
+    'admin_server': '%s/a' % sockdir,
+    'kpasswd_server': '%s/p' % sockdir}}}
 unix_cli = realm.special_env('unix_cli', False, krb5_conf=unix_cli_conf)

 realm.run([kadminl, 'addprinc', '-pw', 'pw1', 'testprinc'])