Regular programs don't usually require these registers to work.
In protected mode, the MOV instructions that access them can only be executed at privilege level 0.
An attempt to read or write the test registers when executing at any other privilege level causes a general protection exception.
Also, those instructions generate invalid opcode exception on most CPUs newer than 80486.
[2] Only register-register moves are supported - as such, the "mod" field (top 2 bits) of the instruction's ModR/M byte should be set to 11b.