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

lldb/test/functionalities/plugins/python_os_plugin/TestPythonOSPlugin.py

@@ -13,7 +13,7 @@ class PluginPythonOSPlugin(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)
 
-    @unittest2.skipUnless(sys.platform.startswith("darwin"), "requires Darwin")
+    @skipUnlessDarwin
     @dsym_test
     def test_python_os_plugin_dsym(self):
         """Test that the Python operating system plugin works correctly"""