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/cpp/dynamic-value/TestCppValueCast.py b/lldb/test/lang/cpp/dynamic-value/TestCppValueCast.py
index e3fbcfc..23a9fc5 100644
--- a/lldb/test/lang/cpp/dynamic-value/TestCppValueCast.py
+++ b/lldb/test/lang/cpp/dynamic-value/TestCppValueCast.py
@@ -13,7 +13,7 @@
     mydir = TestBase.compute_mydir(__file__)
 
     @unittest2.expectedFailure("rdar://problem/10808472 SBValue::Cast test case is failing (virtual inheritance)")
-    @unittest2.skipUnless(sys.platform.startswith("darwin"), "requires Darwin")
+    @skipUnlessDarwin
     @python_api_test
     @dsym_test
     def test_value_cast_with_dsym_and_virtual_inheritance(self):
@@ -31,7 +31,7 @@
         self.setTearDownCleanup(dictionary=self.d_virtual)
         self.do_sbvalue_cast(self.exe_name)
 
-    @unittest2.skipUnless(sys.platform.startswith("darwin"), "requires Darwin")
+    @skipUnlessDarwin
     @python_api_test
     @dsym_test
     def test_value_cast_with_dsym_and_regular_inheritance(self):