[Test] Ensure correct initial datasetSize estimation (elastic#99896)

This PR accounts for the extra file created by ExtraFS for initial
dataset size estimation.

Resolves: elastic#99762

server/src/test/java/org/elasticsearch/index/store/StoreTests.java

@@ -815,7 +815,7 @@ public void testStoreSizes() throws IOException {
         // directory that returns total written bytes as the data set size
         final var directory = new ByteSizeDirectory(StoreTests.newDirectory(random())) {
 
-            final AtomicLong dataSetBytes = new AtomicLong(0L);
+            final AtomicLong dataSetBytes = new AtomicLong(estimateSizeInBytes(getDelegate()));
 
             @Override
             public long estimateSizeInBytes() throws IOException {