Make the home_directory test a little more resilient.

It's possible (albeit strange) for $HOME to intentionally
point somewhere other than the user's home directory as
reported by the password database.  Our test shouldn't fail
in this case.  This patch updates the test to pull directly
from the password database before unsetting $HOME, rather
than comparing the return value of home_directory() to the
original value of the environment variable.

llvm-svn: 298514