Add randomFill to test_utils.h
Summary: Add helper function randomFill to test_utils.h so we can use it in benchmark scrips as well tests.
Test Plan:
```
buck run mode/opt //tvm/sparse:cblas_bench
```
Reviewed By: yinghai
Differential Revision: D17759193
fbshipit-source-id: e4909b04e83ca9382ab4718855fb63743d028de1
