Automate a test to stress eviction walk with many active trees

The eviction server has a limit to the number of trees it can walk, because each walk point requires a hazard pointer and (until SERVER-26545), there was a fixed upper bound on the number of hazard pointers per session.

Add a test with more than 1000 active trees. Eviction should operate fairly so that pages from all trees are equally likely to be evicted.