[lldb-tests] Force system's libcxx on tests failing with debug symbols

The tests in this patch expose failures of LLDBs expression evaluator
when a standard library is compiled with debug symbols. This is the case
for RelWithDebugInfo builds of llvm-project (with libcxx).

Until these bugs are fixed, we force these tests to use the system's
standard library.

Differential Revision: https://reviews.llvm.org/D139361