Fix some lit test ResourceWarnings on Windows
When running LLDB lit tests on Windows, the system selects a debug version
of Python, which was issuing lots of ResourceWarnings about files that
weren't closed. There are two kinds of them, and each test triggered one
This patch fixes one kind by ensuring TestRunner explicitly close the
temporary files created for routing stderr. This is important on Windows
but has no net effect on Posix systems.
The remaining ResourceWarnings are more elusive; the bug may lie in
the Python library subprocess.py, and it may be Windows-specific.
Differential Revision: https://reviews.llvm.org/D63102