Add update_engine_host_unittests

Some of update_engine's codebase (payload generator stuff) run on host.
So it make sense to test them on the host. Adding a new unittest target
update_engine_host_unittests, which includes a subset of unittest. After
this CL, the old update_engine_unittests target remains unchanged, it
still contains all 600 unittests. The new host target will contain a
subset of these.

Test: th
Change-Id: I49c8310cee9a2b2ea197b8ab60ea3b00bf5680cf
diff --git a/payload_consumer/bzip_extent_writer.cc b/payload_consumer/bzip_extent_writer.cc
index 26fdc5f..9491964 100644
--- a/payload_consumer/bzip_extent_writer.cc
+++ b/payload_consumer/bzip_extent_writer.cc
@@ -78,8 +78,7 @@
 
   // Store unconsumed data (if any) in |input_buffer_|.
   if (stream_.avail_in || !input_buffer_.empty()) {
-    brillo::Blob new_input_buffer(input_end - stream_.avail_in, input_end);
-    new_input_buffer.swap(input_buffer_);
+    input_buffer_.assign(input_end - stream_.avail_in, input_end);
   }
 
   return true;