Update PerfRunData.cs

Make CreateDirectory method virtual so that derived classes can provide their own Directory implementation, allowing for benchmarking of custom Directory providers (e.q LiteDB)
diff --git a/src/Lucene.Net.Benchmark/ByTask/PerfRunData.cs b/src/Lucene.Net.Benchmark/ByTask/PerfRunData.cs
index c150602..9aaa449 100644
--- a/src/Lucene.Net.Benchmark/ByTask/PerfRunData.cs
+++ b/src/Lucene.Net.Benchmark/ByTask/PerfRunData.cs
@@ -181,7 +181,7 @@
             SetStartTimeMillis();
         }
 
-        private Store.Directory CreateDirectory(bool eraseIndex, string dirName,
+        protected virtual Store.Directory CreateDirectory(bool eraseIndex, string dirName,
             string dirParam)
         {
             if ("FSDirectory".Equals(config.Get(dirParam, "RAMDirectory"), StringComparison.Ordinal))