jcstress is missing tiers

Readme.md is suggesting:
```
There is a tradeoff between the number of samples harness collects and the suite run time. There are a few preset modes that set sensible test durations, see -m. Many CIs run jcstress with -m quick for quicker turnaround.
```

However -h says:
```
  -iters <N> (Deprecated, to be removed in next releases.)
  -m <mode> (Deprecated, to be removed in next releases).
  -time <ms> (Deprecated, to be removed in next releases.)
```
With time span (based on cores) :
```
1: Total time: 29 minutes [0d+00:29:30]
2: Total time: 16460 minutes [11d+10:20:58]
3: Total time: 33693 minutes [23d+09:33:25]
4: Total time: 149337 minutes [103d+16:57:26]
```

It is becoming to be unbearable to run all tests even on pretty parallel setup.

Jcstress should be able to advice on some kind of tiered testing.