examples: Drop event-test.c dependency on gnulib <verify.h>
examples: Drop event-test.c dependency on gnulib <verify.h>

Pulling in gnulib just for the <verify.h> header is rather
expensive, especially since that header does not require us
to link against gnulib.  It's better to make the event-test
example be standalone by just open-coding a more limited form
of a verify() macro that depends on modern gcc (we have enough
CI coverage that even though the verify is now a no-op in
older setups, we will still notice if we fail to add an event
- as a quick test, I was still able to provoke a compile
failure on Fedora 29 when deleting a line from domainEvents).

Signed-off-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>
Acked-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

andreabolognani/libvirt,nertpinx/libvirt,jfehlig/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,libvirt/libvirt,andreabolognani/libvirt,zippy2/libvirt,libvirt/libvirt,andreabolognani/libvirt,nertpinx/libvirt,zippy2/libvirt,eskultety/libvirt,eskultety/libvirt,libvirt/libvirt,crobinso/libvirt,zippy2/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,eskultety/libvirt,eskultety/libvirt,jfehlig/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,olafhering/libvirt,andreabolognani/libvirt,nertpinx/libvirt,nertpinx/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,jfehlig/libvirt,jardasgit/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,nertpinx/libvirt,olafhering/libvirt,fabianfreyer/libvirt,olafhering/libvirt,crobinso/libvirt,crobinso/libvirt,eskultety/libvirt,crobinso/libvirt