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/macosx/add-dsym/TestAddDsymMidExecutionCommand.py b/lldb/test/macosx/add-dsym/TestAddDsymMidExecutionCommand.py
index 22108f5..29ab55e 100644
--- a/lldb/test/macosx/add-dsym/TestAddDsymMidExecutionCommand.py
+++ b/lldb/test/macosx/add-dsym/TestAddDsymMidExecutionCommand.py
@@ -6,7 +6,7 @@
 import sys
 from lldbtest import *
 
-@unittest2.skipUnless(sys.platform.startswith("darwin"), "requires Darwin")
+@skipUnlessDarwin
 class AddDsymMidExecutionCommandCase(TestBase):
 
     mydir = TestBase.compute_mydir(__file__)