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

[kastiglione]

Summary

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++.

[JDevlieghere]

LGTM

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

def do_test_with_run_command(self, stdlib_type):
 
         self.runCmd("frame variable numbers_list --show-types")
 
         self.runCmd("type format add -f hex int")
 
         self.expect("frame variable numbers_list --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',
                              '{}'])
 
         self.expect("p numbers_list",
                     substrs=['size=0',
                              '{}'])