apple hardware test disappeared from my mac

I have a macbook air 11 inch Mid 2013 running 10.10.5 (Yosemite).

Pressing D or option+D on reboot does not start the Apple Hardware Test.

Searching for this topic in the internet, some people mentioned the

/System/Library/CoreServices/.diagnostics. In fact, this directory or file

is not present in my mac.

(1) Is this the reason the hardware test will not start?

(2) How do I get this file or directory back on my mac?

May mac is MacBookAir6,1, Board Mac-35C1E88140C3E6CF.

Thanks,

Marcia