Clean up printing of testset start (#33408)
This has bothered me for a while, but looks like it didn't bother
anyone else enough to clean it up, so here I go :). Printing when
a test starts is useful when looking through logs because you can
correlate errors on a given worker with the specific test it was
running. However, it doesn't have to be so ugly and break the nice
regular formatting of the rest of the test.
Before:
```
Test (Worker) | Time (s) | GC (s) | GC % | Alloc (MB) | RSS (MB)
From worker 3: running testset int...
From worker 2: running testset math...
int (3) | 9.51 | 0.21 | 2.2 | 507.51 | 232.42
math (2) | 81.06 | 0.75 | 0.9 | 3391.19 | 323.18
Test Summary: | Pass Total
Overall | 1637318 1637318
SUCCESS
```
After:
```
Test (Worker) | Time (s) | GC (s) | GC % | Alloc (MB) | RSS (MB)
int (3) | started at 2019-09-27T18:35:02.761
math (2) | started at 2019-09-27T18:35:02.883
int (3) | 11.91 | 0.28 | 2.4 | 723.85 | 278.07
math (2) | 40.51 | 0.87 | 2.1 | 3287.85 | 325.49
Test Summary: | Pass Total
Overall | 2030534 2030534
SUCCESS
```
