Been doing FHA appraisals for over 30 years and unless something changed that I am not aware of, we are to attest that “all fixtures and appliances appear to be in working order”. That is the verbiage I put in the report. The only “sample” tests are for electrical outlets and built in light fixtures. I turn on all faucets, including shower heads and flush all toilets. If I can’t because some are turned off, I make it subject to.
I have done inspections in the past that are similar to yours and ask the realtor or owner why they are turned off and ask permission if I can turn them on. Sometimes you just have to open the valve at the fixture. Always ask, though!