libc++ testing: fix assertion around `use_system_cxx_lib`

Actually fix (hopefully) the assertions about `use_system_cxx_lib`,
the previous attempt failed because I misread the error.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@299056 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed