Is that legal for a large diagnostic company to lay off techs and then rehire them as independent contractors and pay them per test done instead of per/hour?
Considering that the company still dictates when the techs have to come to work and how many tests they should perform per day/

sure its legal a company can fire and hire whomever they please whether its the same people or not.
The company was trying to cut costs and by having independent contractors the company doesnt pay benefits.......vacation pay.health care.....sick time

IF they dictate the hours and terms of work, then the techs are NOT independent contractors; they are employees.

yes it very legal it happening all the time to cut cost