VB: add test for iteritems performance
catch the regression noted in #2273 next time.

```
frame_iteritems                           24.85       3.12  7.98

Columns: test_name | head_time [ms] | baseline_time [ms] | ratio

- a Ratio of 1.30 means HEAD is 30% slower then the Baseline.

Head [59d318c] : revert
Baseline [bdbca8e] : BUG: fix borked data-copying iteritems performance affecting DataFrame and SparseDataFrame. close #2273
```
