OK, just checking, as there are a myriad of techniques.
I think the answer to this lies within your own organization, as there is no industry "best practice" for this (VMware suggest mixing workloads, as long as your an in compliance...).
If your test VMs access different networks to production (highly likely) then there's not problem mixing them in a cluster that has access to both networks (e.g using VLANs) and using resource pools (test vs prod) to separate and protect resources, but be careful about VC access - this is the biggest danger, letting "test" folks access the same system as "production".
This is why I think the answer lies in your organization - does your current compliance insist on physical separation of test and prod, including administrative duties? These are the kind of compliance issues that dictate your answer.
HTH
Steve
PS. There is immense value in sharing a cluster between dev, test, prod, because you have the great mix of different workloads with protection from DRS plus high availability for dev/test which are more important than folks always think (to the business during a release anywayz!). Be aware of luddites who don't have a strong reason for mixing other than technical/process rigidity. These are the same people who will never have changes in business hours, never do the risk calculation, and stangle the business by insisting on 5 day change windows for a vMotion! 