Update TestValidation/Timestamp_Validation_test_6

TestValidation/Timestamp_Validation_test_6 is broken. We should fix or remove.

```
=== RUN   TestValidation/Timestamp_Validation_test_6
    eval_test.go:349: Compile() got error = unknown name timestamp (1:1)
         | timestamp(x)
         | ^, wantErr false
    --- FAIL: TestValidation/Timestamp_Validation_test_6 (0.00s)
```