If you want to test IAC Valve remove it
Then plug in its 2 wire connector
Turn key ON, engine off
Look inside at the valve part of IAC, it should be fully OPEN with key on engine off
Now unplug the 2 wire connector, it should close, only moves 3/8" but MOVES
Plug wires back in and it should open again
Repeat until you confirm it is, OR it IS NOT, moving
And no its metal case does not need to be grounded to engine for it to work, it has 2 wires so + and - are done with those
One of the failure modes of an IAC valve is a short in one contact to its metal case, you can and should test for that with OHM meter, this type of short causes IAC Valve to open and stay open when key is on, and it will test as OK.
Use ONLY Motorcraft IAC Valve as replacement, there have been numerous issues with 3rd party IAC Valves
An old wrecking yard Motorcraft IAC Valve would be better than new 3rd party