[libc++][test] Remove some `XFAIL` for LLVM libc in multi-threaded tests (#228036)

Recently, LLVM libc fixed https://llvm.org/PR194029 in
https://llvm.org/PR226134, so more libc++ tests passes with LLVM libc
now.

Co-authored-by: Aiden Grossman <aidengrossman@google.com>
GitOrigin-RevId: 0791157af02871142368d001fd8c2c1fd2b3a2a6