Test kit chemicals in so little quantity (Concentrations) will not do any harm to your pipes/sewer lines especially because you are diluting with tap water. To my knowledge, even if test kit chems are dumped down the drain one right after the other, they will not react (if any) to the extent that they will cause damage to pipes. So, as long as you are diluting with running fresh water, You R Safe
Just be careful when dumping High Acidic (Muratic Acid) or High Base (Red Devil Lye) down the drain in large quantities though - Just a warning from Personal Experience
Just My $0.02