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 of each.
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.