When dropping privileges to run tests, ENV["USER"] could be set
to a user that doesn't match Process.euid, which causes this
test to fail with Errno::EPERM.  Try to get the name for the
current euid, and only fallback to ENV["USER"] if that doesn't
work.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66428 b2dd03c8-39d4-4d8f-98ff-823fe69b080e