handler: Implement open as a pass-through function

When opening a blob id, handler finds a blobstore with matching
base id and calls its open function, passing down the path and open
flags.

Signed-off-by: Kun Yi <kunyi@google.com>
Change-Id: I6127b3c2b2c4752b5e094db61f12451089ccb20b
diff --git a/test/handler_unittest.hpp b/test/handler_unittest.hpp
index f6390fc..7f5df42 100644
--- a/test/handler_unittest.hpp
+++ b/test/handler_unittest.hpp
@@ -15,4 +15,8 @@
     BinaryStoreBlobHandler handler;
 };
 
+class BinaryStoreBlobHandlerOpenTest : public BinaryStoreBlobHandlerTest
+{
+};
+
 } // namespace blobs