tests: don't set G_DEBUG=fatal-warnings on macOS

tests: don't set G_DEBUG=fatal-warnings on macOS

On macOS when BROKEN_POLL is set in GLib, our tests will periodically
trigger a warning:

  (process:50880): GLib-WARNING **: 02:54:15.272: poll(2) failed due to: Bad file descriptor.

Our code is inherantly racy, calling g_source_destroy which
removes the FD from the event thread poll asynchronously but
we close the FD immediately after g_source_destroy returns.

With poll() this results in POLLNVAL which we're ignoring, but
with select() it generates the BADF error on macOS.

We need to ignore the warnings on macOS to avoid abort()ing
our test programs.

Closes: https://gitlab.com/libvirt/libvirt/-/issues/303
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>