Stop relying on $HOME being set in the unit tests

When I test Tor with $HOME unset, I see the following error:

options/validate__uname_for_server: [forking] 
  FAIL ../src/test/test_options.c:507: expected log to not contain entries  Captured logs:
    1. warn: "Couldn\'t find $HOME environment variable while expanding \"~/.tor\"; defaulting to \"\".\n"
    2. warn: "Default DataDirectory is \"~/.tor\".  This expands to \"/.tor\", which is probably not what you want.  Using \"/usr/local/var/tor\" instead\n"

  [validate__uname_for_server FAILED]

This test has been around since 0.2.8.1-alpha.