tests: fix deadlock in eventtest

tests: fix deadlock in eventtest

There is a race deadlock in eventtest after the recent rewrite to drop
GNULIB from libvirt code base.

The issue happens when the callbacks testPipeReader() or testTimer()
are called before waitEvents() starts waiting on `eventThreadCond`.
It will never happen because the callbacks are already done and there
is nothing that will signal the condition again.

Reported-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>