Update TestEval test case "strings"

TestEval "strings" is broken. We should fix or remove.

```
eval_test.go:169: Eval() got error = failed to evaluate: invalid operation: int(string) (1:12)
         | object.abc.join(', ')
         | ...........^, wantErr false
    --- FAIL: TestEval/strings (0.00s)
```