spec/tools: allow launching from any directory

So that this works
  export BUNDLE_GEMFILE=Gemfile.ci
  ./spec/tools/test_env bundle exec rake bare:spec
while
  bundle exec rake test
says
  Bundler::GemfileNotFound: .../ruby-dbus/spec/tools/Gemfile.ci not found
Redirect dbus-daemon output.

otherwise dbus-daemon will write "Activating service foo" to a
descriptor that has been closed and will die on SIGPIPE.