tests: don't try to mock __open_2 on non-GLibc builds

tests: don't try to mock __open_2 on non-GLibc builds

Mocking of the __open_2 function was added in

  commit 459f071cacf30af9df93b7d090b1bda71b0ef20f
  Author: Michal Privoznik <mprivozn@redhat.com>
  Date:   Thu Aug 15 16:37:17 2019 +0200

    virpcimock: Mock __open_2()

This function only exists in glibc, however, and the mocking code runs
on systems not using glibc, such as FreeBSD. Even Linux hosts might be
using a different libc impl, though we don't actively try to support
that.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>