[lldb][test] Skip TestStdFunctionStepIntoCallable.py

With swiftlang/swift#66018 we started to run standalone tests
(e.g., the ones on swift-ci) against newly built libcxx.
This caused the test to fail on the 5.9 branch. The first
step-in into a pointer to a data member didn't behave as expected.

Thus skip it for now.

(cherry picked from commit dbd0474)

Author: Michael137

lldb/test/API/lang/cpp/std-function-step-into-callable/TestStdFunctionStepIntoCallable.py

@@ -12,6 +12,7 @@
 class LibCxxFunctionSteppingIntoCallableTestCase(TestBase):
     NO_DEBUG_INFO_TESTCASE = True
 
+    @skipIfDarwin
     @add_test_categories(["libc++"])
     def test(self):
         """Test that std::function as defined by libc++ is correctly printed by LLDB"""