Which test was conducted on the skin found under the victim's nails

DNA testing

Explanation:

Under the field of forensic science DNA testing or DNA evidence is considered as the most reliable source of getting accurate result. Decades of rigorous scientific study has made the DNA a golden strand of forensic science.

For example, this test can find out the presence of person at the crime scene, even a small sample of semen, blood, hair or skin can give out more information than any other test.