check for platform-specific env. variables

'USERNAME' is not a standard environment variable outside of Windows,
which leads to this test failing on non-Windows Meterpreter platforms.

test/modules/post/test/get_env.rb

@@ -53,8 +53,12 @@ class MetasploitModule < Msf::Post
 
   def test_get_envs
     it "should return multiple envs" do
-      res = get_envs('PATH','USERNAME')
-      !res['PATH'].blank? && !res['USERNAME'].blank?
+      res = get_envs('PATH','USERNAME','USER')
+      if session.platform =~ /win/i
+        !res['PATH'].blank? && !res['USERNAME'].blank?
+      else
+        !res['PATH'].blank? && !res['USER'].nil?
+      end
     end
   end