misc/tst-preadvwritev2 incorrect test of [BZ #23579"]

[Florian Weimer]

* Sergey Kolosov:

> glibc misc/tst-preadvwritev2 and misc/tst-preadvwritev64v2 tests are
> always passing even on unpatched glibc. I've found out that added test
> case function do_test_with_invalid_fd() checks preadv2 (-1, &iov, 1, 0,
> RWF_HIPRI) result with TEST_VERIFY (errno == EBADF || errno ==
> ENOTSUP), that means both values(correct EBADF and incorrect ENOTSUP)
> are considered valid, so the test is always passing.

I think we need to probe first if there is emulation or not, and skip
the test if there is kernel support.

Do you want to work on a patch?

Thanks,
Florian