Replace sys.platform skips in tests with @skip decorators which check against remote platform.

Adds @skipIfPlatform and @skipUnlessPlatform decorators which will skip if /
unless the target platform is in the provided platform list.

Test Plan:
ninja check-lldb shows no regressions.
When running cross platform, tests which cannot run on the target platform are
skipped.

Differential Revision: http://reviews.llvm.org/D8665

llvm-svn: 233547
diff --git a/lldb/test/lang/objc/self/TestObjCSelf.py b/lldb/test/lang/objc/self/TestObjCSelf.py
index cc58bf2..1ed2283 100644
--- a/lldb/test/lang/objc/self/TestObjCSelf.py
+++ b/lldb/test/lang/objc/self/TestObjCSelf.py
@@ -9,14 +9,14 @@
     
     mydir = TestBase.compute_mydir(__file__)
     
-    @unittest2.skipUnless(sys.platform.startswith("darwin"), "requires Darwin")
+    @skipUnlessDarwin
     @dsym_test
     def test_with_dsym_and_run_command(self):
         """Test that the appropriate member variables are available when stopped in Objective-C class and instance methods"""
         self.buildDsym()
         self.self_commands()
 
-    @unittest2.skipUnless(sys.platform.startswith("darwin"), "requires Darwin")
+    @skipUnlessDarwin
     @dwarf_test
     def test_with_dwarf_and_run_command(self):
         """Test that the appropriate member variables are available when stopped in Objective-C class and instance methods"""