Add unit tests to test delta performer directly.

Also changed TEST to TEST_F to make life easier.

Bug: 23720829
TEST=cros_workon_make update_engine --test

Change-Id: I1040af91de125399c7b90255f8f0996afe545e1f
diff --git a/delta_performer.cc b/delta_performer.cc
index baeef08..4b1981b 100644
--- a/delta_performer.cc
+++ b/delta_performer.cc
@@ -283,7 +283,7 @@
 
 int DeltaPerformer::Close() {
   int err = 0;
-  if (!kernel_fd_->Close()) {
+  if (kernel_fd_ && !kernel_fd_->Close()) {
     err = errno;
     PLOG(ERROR) << "Unable to close kernel fd:";
   }