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/python_api/section/TestSectionAPI.py b/lldb/test/python_api/section/TestSectionAPI.py
index de13717..6c7b4f5 100755
--- a/lldb/test/python_api/section/TestSectionAPI.py
+++ b/lldb/test/python_api/section/TestSectionAPI.py
@@ -9,7 +9,7 @@
 
     mydir = TestBase.compute_mydir(__file__)
 
-    @unittest2.skipUnless(sys.platform.startswith("darwin"), "requires Darwin")
+    @skipUnlessDarwin
     @python_api_test
     @dsym_test
     def test_get_target_byte_size_with_dsym(self):