Added basic span reader test.

Might as well right better tests when writing sorting v2.  Backfilling
is needed here before we can build up further.

Change-Id: Ibe805cb031eba3eac4bb3d65dfa318c52e6487e2
diff --git a/aos/testing/BUILD b/aos/testing/BUILD
index ce67b84..e70b5a4 100644
--- a/aos/testing/BUILD
+++ b/aos/testing/BUILD
@@ -86,3 +86,11 @@
     ],
     visibility = ["//visibility:public"],
 )
+
+cc_library(
+    name = "tmpdir",
+    testonly = True,
+    srcs = ["tmpdir.cc"],
+    hdrs = ["tmpdir.h"],
+    visibility = ["//visibility:public"],
+)