[test] Fix propagating HOME envvar to unittests

Propagate HOME environment variable to unittests.  This is necessary
to fix test failures resulting from pw_home pointing to a non-existing
directory while being overriden with HOME.  Apparently Gentoo users
hit this sometimes when they override build directory for Portage.

Original bug report: https://bugs.gentoo.org/674088

Differential Revision: https://reviews.llvm.org/D56162

llvm-svn: 350175