Increase format stress testing scope.

Summary:

Increase the length of stress runs, setting a minimum of 8 minutes and a maximum of 30 minutes.

When we created CONFIG.stress, we increased the standard run timer to 4 minutes, and we should increase that further. I was thinking of adding a new configuration that would replace "timer" in CONFIG.stress and would run somewhere between 1 and 30 minutes, with a distribution heavily weighted to running somewhere between 4 and 10 minutes, but occasionally doing a much longer run.
 
We have things we want to test that take longer than 4 minutes to fire (for example, and the reason I've been thinking about this, the effect of an open backup cursor on checkpoint deletion).