Test refactor: minimize number of created collections

[vkarpov15]

Now that mmapv1 is gone, we have to use WT for tests, and the problem there is that dropDatabase() is very slow on WT, particularly if you're running a replica set and have a lot of collections. On slower machines, it can take over a minute to drop all the collections our tests create. In order to make tests more portable and minimize the amount of tuning we need to do to run the tests, we should reuse collections where possible, and drop() collections in between individual tests so we don't need to wait for an entire dropDatabase().

[vkarpov15]

Also see d938e9a