[lldb] [test] Skip ObjC-based tests via 'objc' category

Replace the plethora of ObjC-implied 'skipUnlessDarwin' decorators
with marking tests as 'objc' category (whenever missing), and skip all
ObjC tests on non-Darwin platforms.  I have used '.categories' file
wherever it was present already or all (>1) tests were relying on ObjC,
and explicit add_test_categories() where there was only one test.

Differential Revision: https://reviews.llvm.org/D91056

lldb/test/API/python_api/objc_type/TestObjCType.py

@@ -19,8 +19,7 @@ class ObjCSBTypeTestCase(TestBase):
         TestBase.setUp(self)
         self.line = line_number("main.m", '// Break at this line')
 
-    @skipUnlessDarwin
-    @add_test_categories(['pyapi'])
+    @add_test_categories(['objc', 'pyapi'])
     def test(self):
         """Test SBType for ObjC classes."""
         self.build()

lldb/test/API/python_api/sbtype_typeclass/TestSBTypeTypeClass.py

@@ -3,7 +3,7 @@ from lldbsuite.test import lldbinline
 
 lldbinline.MakeInlineTest(
     __file__, globals(), [
-        decorators.skipUnlessDarwin,
+        decorators.add_test_categories(["objc"]),
         decorators.expectedFailureAll(
             oslist=['macosx'], archs=['i386'],
             bugnumber='rdar://28656677')])