Fixed up intermittently failing tests to skip on Linux.

Also added 'import sys' on some tests that are using non-standard
unittest2.skipUnless blocks with code that is intended to do things
that we have more specializes @* attributes for.  These skip
conditions were failing to execute due to missing import, causing
darwin-only tests to run on Linux regardless.  Will file a bug for
that separately.

llvm-svn: 204747
diff --git a/lldb/test/macosx/add-dsym/TestAddDsymMidExecutionCommand.py b/lldb/test/macosx/add-dsym/TestAddDsymMidExecutionCommand.py
index e74c37f..1fc8e59 100644
--- a/lldb/test/macosx/add-dsym/TestAddDsymMidExecutionCommand.py
+++ b/lldb/test/macosx/add-dsym/TestAddDsymMidExecutionCommand.py
@@ -4,6 +4,7 @@
 import unittest2
 import lldb
 import pexpect
+import sys
 from lldbtest import *
 
 @unittest2.skipUnless(sys.platform.startswith("darwin"), "requires Darwin")