Perf_Directory cleanup fix (dotnet#405)

Fix cleanup in test initialization

Author: billwert

src/benchmarks/micro/corefx/System.IO.FileSystem/Perf.Directory.cs

@@ -67,7 +67,7 @@ public IEnumerable<object> RecursiveDepthData()
                             var longPath = GetDirectoryPath(_testFile, (int)depth);
 
                             Directory.CreateDirectory(longPath);
-                            Directory.Delete(longPath);
+                            Directory.Delete(_testFile, recursive: true);
 
                             return true;
                         }