Print the execution time for each test in sub_test

I want to track down the execution time for our multi-locale perf runs, and I
think this will be useful to others too (I've found the total time for sub_test
being printed to be useful.)
Or maybe with the execution word:

```
[Elapsed execution time for "studies/filerator/walk (execopts: 9)" - 1.123 seconds]
```