[lldb][test] TestVTableValue.py: skip base_class_ptr test case on older Clang versions

This failed on the public LLDB matrix build bots on Clang versions <
9.0:
```
======================================================================
FAIL: test_base_class_ptr (TestVTableValue.TestVTableValue)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "lldb/test/API/functionalities/vtable/TestVTableValue.py", line 90, in test_base_class_ptr
    self.assertEquals(shape_ptr_vtable.GetNumChildren(), 5)
AssertionError: 6 != 5
```