test: fix flaky viny/tx_gap_lock test

The `cmp_tuple` helper function is broken - it assumes that all tuple
fields, including the payload, are numeric. It isn't true - the payload
field is either nil or string. This results in a false-positive test
failure:

```
error: '[string "function cmp_tuple(t1, t2)     for i = 1, PAY..."]:1:
       attempt to compare nil with string'
```

Closes #6336

NO_DOC=test
NO_CHANGELOG=test