Fix issues reported by staticcheck in tests

[shane-ns1]

Fixes #25

This PR resolves all of the issues reported by the staticcheck tool in tests.

I think it's mostly boring, except possibly this removed code in BenchmarkTxn_Sub_abort():

                for i := 0; i < b.N; i++ {
                        txn.Sub(func(txn *Txn) (err error) { return e })
-                       if e == nil {
-                       }
                }

I'm not really sure what the intention is, since e never changes. If the idea was to add some actual check, it was never added...

[wojas]

Thanks, this all looks better or harmless and does clean things up!