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.)
I would prefer the test name here and some kind of separator between it and the time (for easy parsing). e.g.

```
[Elapsed time for "users/ferguson/fielderator" - 1.123 seconds]
```

or with execopts:

```
[Elapsed time for "studies/filerator/walk (execopts: 9)" - 1.123 seconds]
```

Also, the per-sub_test time is reported with 3digit precision (`{1!.3f}` is the pattern), so it might be good to keep them the same (2 or 3 is fine by me).