Treat root as a valid candidate for TEST_USER_ME

Treat root as a valid candidate for TEST_USER_ME

If spawn_dbus_daemon() can fail for TEST_USER_ME, then we'd have to
go through all the tests adding the ability to skip tests after
it fails, which is a fairly extensive change.

The tests have historically all run as whatever uid is supplied, and
if the tests are being run as root for some reason - perhaps in a CI
framework for an embedded platform that doesn't have non-root users,
or in an environment where you can be root or non-root but not both -
there is no particular reason to skip them.