Fix LinkTest.OldnameDoesNotExist
Fix LinkTest.OldnameDoesNotExist

Previous, this test was the same as OldnameIsEmpty. This CL makes the test check
what happens if the old name does not exist.

PiperOrigin-RevId: 390243070

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor