When you are trying to run setup /testdbupgrade and you get an error asking for a native client (because you're running the command outside of your DB site or just doesn't have this particular cli that SCCM db test is looking for), you should grab the sqlncli.msi (x64) from Microsoft SQL Server 2012 Feature Pack. Or you can grab it from running setup to get that as a prereq download ($SCCMInstallISORoot\SMSSETUP\BIN\X64\SetupDL.exe).

Also, if you're having trouble pointing to your SQL server using SERVER\dbinstance in your testdbupgrade switch, you can try setup /testdbupgrade $dbinstance /testsqlserver $SERVER