docs: Added screen zoom testing info.

I'd pulled the "testing screen zoom" material out of the draft doc,
since we were planning on putting it in a separate "testing" doc.
Since we don't have a testing doc, I'm restoring it here.

See first comment for doc stage location.

Change-Id: Ie1375cd4d61f92ab7a75b2dba2d31b340c7244d5