I would say: letter of employment
A testimonial usually consists of positive comments about a product or service.
In Canada, possibly also in the US (not sure), letters of referral, certainly letters of recommendation, are rarely given these days AFAIK.
I worked in the corporate world for a long time. Letters of referral/recommendation were discontinued a long time ago due to the possibility of error. (i.e., Someone may have been a fair or even great employee for you, but fail miserably in their new job .... ).
The letters given by employers now usually consist of: XYZ worked for ABC corporation from xx/xx/xx to xx/xx/xx. Period.