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

[Sergey Kolosov]

Hello,

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.

Thanks,
Sergey