Add test to sync 100,000 new files

[gdt]

We have occasional reports of running out of memory. It would be nice if the automated tests did something stressful. This ticket is just to add something that is a) stressful and b) we think can fit in some reasonable amount of RAM that we document in the test instructions.

However, that might cause more trouble than it is worth. So it might lead to test options where the normal "make test" does not run tests that are expensive.