virshtest: Adapt 'event' option arg handling tests from 'virsh-optparse'

virshtest: Adapt 'event' option arg handling tests from 'virsh-optparse'

Move the argument parsing tests excercising 'virsh event' options
from 'virsh-optparse' to 'virshtest'.

As the test invokes 'virsh event' with a timeout and thus waits for one
second pointlessly the patch also adds infrastructure to mark individual
cases as expensive and is skipped normally.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>