Remove the clear_dirty_log test, instead merge it into the existing
dirty_log_test.  It should be cleaner to use this single binary to do
both tests, also it's a preparation for the upcoming dirty ring test.

The default behavior will run all the modes in sequence.
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20201001012233.6013-1-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>