BaseDirectoryTestCase should use try-with-resources for its Directories [LUCENE-6916]

I'm playing around with writing a nio2 FileSystem implementation for HDFS that will work with Directory, and it currently leaks threads everywhere because if a BaseDirectoryTestCase test fails it doesn't close its Directory. This obviously won't be a problem if everything passes, but it will probably be a while before that's true and it makes iterative development a bit of a pain.

---
Migrated from [LUCENE-6916](https://issues.apache.org/jira/browse/LUCENE-6916) by Alan Woodward (@romseygeek)
Attachments: [LUCENE-6916.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-6916/LUCENE-6916.patch)