Testing speed up: bulk insert Predictions

3 test cases were inserting several thousand Prediction objects one at a
time. This does bulk insertion. See timing results below:

```
real    4m17.027s
real    0m28.400s
```

Result! [Delivers #76460256]
Merge pull request #78 from sealevelresearch/speed-up-tests

Testing speed up: bulk insert Predictions