Don't delete test directories after successful tests.

We have tests that are flaky because of failures to delete files on
Windows. This is caused by a problem with Windows/Java where files that
have been memory-mapped have locks held longer than expected (see
http://bugs.java.com/bugdatabase/view_bug.do?bug_id=4715154). We can
work around this problem completely by not even trying to delete the
test directories.

This means that subsequent runs of the tests will start with dirty
directories. There is an earlier commit which changes all our tests to
clean out the directories that they use before they start running the
tests.
Merge pull request #2210 from benbc/2.0-dont-delete-test-directories

2.0 dont delete test directories