disable test_thread_exit on rustc >= 1.48
disable test_thread_exit on rustc >= 1.48

This has regressed since 1.48 and no longer passes.  Unwinding across
ABI boundaries is still undefined until the "C-unwind" project is
complete (see https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md)

dgrunwald/rust-cpython,dgrunwald/rust-cpython