A few more fixes, and test fastdecode under Kokoro.

BUILD

diff --git a/BUILD b/BUILD
index 3789b98..1276714 100644
--- a/BUILD
+++ b/BUILD
@@ -38,7 +38,7 @@
 
 upb_fasttable_enabled(
     name = "fasttable_enabled",
-    build_setting_default = True,
+    build_setting_default = False,
     visibility = ["//visibility:public"],
 )