A little late to the party, but same advice as above...
In my experience, all homes have some sort of mold, and mold testing companies will find it -- so there is that portion of it.
I think it is important to distinguish that you want a mold testing company that ONLY tests for mold, NOT a mold remediation company that is highly interested in remediating that mold. Conflict of interest is apparent...
You, not the tenant, hire the company to test.
As previously mentioned, tenants wanting to break a lease, love to lean on mold as an excuse, because they know there's a good argument for it on their side. I am not sure of your relationship with the tenants, etc. but a lot goes into this. Personally, with the market the way it is, I would lean towards letting them out of their lease, as you may get a rent increase -- bit of a win/win.
If you have any questions, feel free to DM me...happy to help, and pretty well versed in Landlord/Tenant law (disclaimer** not admitted to practice law in GA) -- as I went to law school in FL.