[libc++][test] Refactor SmallBasicString uses in range.lazy.split tests

The tests for `std::ranges::lazy_split_view` heavily use a wrapper class around
`std::string` because `std::string` was not `constexpr` until recently. Where
possible, remove the wrapper class and extra functionality no longer needed.
Remove `libcxx/test/std/ranges/range.adaptors/range.lazy.split/small_string.h`
and inline its one use remaining in
`libcxx/test/std/ranges/range.adaptors/range.lazy.split/general.pass.cpp`.

Differential Revision: https://reviews.llvm.org/D126663