Summary:
Make DeleteFileTest inherit DBTestBase to avoid code duplication.

Test Plan (on devserver)
```
$make deletefile_test
$./deletefile_test
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5822

Differential Revision: D17456750

Pulled By: riversand963

fbshipit-source-id: 224e97967da7b98838a98981cd5095d3230a814f