[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