Remove all files created by tests in a {{post}} task

In Evergreen, tests should remove the files they create using a post command, possibly in addition to in a pre command.
Currently, the cleanup happens in a pre command, but some projects don't run a cleanup in a pre command, and can have some trouble with files being left from a previous task.

For example, this test filled up /data slice with 37G of data that caused disk space issues for the next tasks that were executed on the same box - https://evergreen.mongodb.com/task/mongo_tools_v3.2_ubuntu_race_qa_tests_885f45c3681e829f236784beb43cee902c9b4cd1_16_05_24_15_20_05