fix(busted) catch and propagate tests exit code
fix(busted) catch and propagate tests exit code

LuaJIT's OpenResty is compiled with Lua 5.2 compatibility, which changes
the return values of `os.execute()`. Once again, this breaking changes
messes with us; it seems like this change will haunt us forever, whether
in Kong or in the many third-party libraries we rely upon and already
pushed fixes for... :sigh:

Kong/kong,Kong/kong,jebenexer/kong,Mashape/kong,Kong/kong