The instruction is encoded in two ways, depending on the flow of data. Moving data from a general purpose register into a test register is encoded as 0F 26 /r (with r/m being the GPR, and reg being the test register).
The encoding of this instruction depends on the direction of data flow. To move data from a general-purpose register into a test register, use the code 0F 26 /r, where r/m specifies the source and reg specifies the destination.