point $HOME to empty tempdir rather than a nonexistent path

Created originally on Bitbucket by kelledin (Frank Schaefer)

Was already merged in Bitbucket before import, marked as merged by the import user

Lately I had to deal with test_distutils_* and test_setuptools_* failing when the testcases tried to run setup.py. It turns out that setting $HOME to a nonexistent directory was causing ccache to face-plant, which naturally rolled downhill to break the setup.py run. It seems to me that the cleanest way to resolve this is to create an empty tempdir to use as $HOME for each testcase.

(FYI I normally have symlinks set up so ccache gets run silently on every gcc/g++ invocation. I believe Debian pybuild also likes to use ccache in the background, although I haven’t dug into the low-level details.)