Start DeleteFileTest with clean plate

Summary:
Remove all the files from the test dir before the test. The test failed when there were some old files still in the directory, since it checks the file counts.
This is what caused jenkins' test failures. It was running fine on my machine so it was hard to repro.

Test Plan:
1. create an extra 000001.log file in the test directory
2. run a ./deletefile_test - test failes
3. patch ./deletefile_test with this
4. test succeeds

Reviewers: haobo, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14097