[lldb][test] Combine libstdc++ and libc++ iterator tests into generic test (#147175)
This combines the libc++ and libstdc++ test cases. The libstdcpp tests
were a subset of the libc++ test, so this patch moves the libcxx test
into generic and removes the libstdcpp test entirely.
There are currently no formatters for libstdcpp
std::unorderd_map::iterator. So I removed those test-cases. We already
test them for libc++ in `libcxx/unordered_map-iterator`. And we test
`std::unordered_map` in `generic/unorderd`. So these test-cases would be
redundant.
Split out from https://github.com/llvm/llvm-project/pull/146740