[libcxx][lit] Fix running testsuite with python2.7 after 9020d286

Python 2.7 fails with TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'
if you pass None as the prefix argument to NamedTemporaryFile.

Reviewed By: ldionne, bjope, #libc

Differential Revision: https://reviews.llvm.org/D84595