[lldb] Guard libstdc++ specific 'frame var' test

While working on D116788 (properly error out of `frame var`), this libstdc++
specific `frame var` invocation was found in the tests. This test is in the
generic directory, but has this one case that requires libstdc++. The fix here
is to put the one `expect()` inside of a condition that checks for libstdc++.

Differential Revision: https://reviews.llvm.org/D116901

Author: kastiglione

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/list/TestDataFormatterGenericList.py

@@ -62,12 +62,14 @@ def cleanup():
         self.expect("frame variable numbers_list --raw", matching=False,
                     substrs=['size=0',
                              '{}'])
-        self.expect(
-            "frame variable &numbers_list._M_impl._M_node --raw",
-            matching=False,
-            substrs=[
-                'size=0',
-                '{}'])
+
+        if stdlib_type == USE_LIBSTDCPP:
+            self.expect(
+                "frame variable &numbers_list._M_impl._M_node --raw",
+                matching=False,
+                substrs=[
+                    'size=0',
+                    '{}'])
 
         self.expect("frame variable numbers_list",
                     substrs=['size=0',