Fix Coverity warnings about mkstemp in tests
umask(2) should always be used prior to mkstemp(3) so the temporary file
won't be created with insecure permissions.
Reported by: Coverity
CID: 1331605 1347173 1375366 1339800 1331604 1296056 1296060
CID: 1296057 1296062
MFC after: 2 weeks