It depends on the manufacturer. All do some testing, some do a lot of testing. Spyderco in particular has delayed the release of new models for years at times before they got everything right. They design, make prototypes, mechanically break them, refine, prototype and break until it meets their criteria. Once they reach that point, they get field tested. In the case of collaboration models, the co-designer gets prototypes that have passed all other testing to use and evaluate. He then sends them back with any refinements he feels are needed, and more prototypes are made and tested until both Sal and the other designer are satisfied. I may be remembering wrong, but I believe it took three years to finalize the design of the BushcraftUK.
In the case of in-house designs, Sal does a lot of field testing himself, and I believe employees do as well. Sometimes pilot run (pre-production but final design) knives get sent to lucky customers for evalution and photographs before they are officially released.