Fix test failure if ENV["USER"] doesn't match Process.euid

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

test/ruby/test_process.rb

@@ -1567,7 +1567,7 @@ class TestProcess < Test::Unit::TestCase
   end
 
   def test_seteuid_name
-    user = ENV["USER"] or return
+    user = (Etc.getpwuid(Process.euid).name rescue ENV["USER"]) or return
     assert_nothing_raised(TypeError) {Process.euid = user}
   rescue NotImplementedError
   end