Skip preallocate tests on windows (elastic#110998)

The preallocate tests assumed that preallocation was using the fallback
implementation which calls setLength on Windows. However, that fallback
only happens inside the SharedBytes class, so windows doesn't actually
do anything when tryPreallocate is called. This commit skips the test on
windows.

closes elastic#110948

libs/native/src/test/java/org/elasticsearch/nativeaccess/PreallocateTests.java

@@ -18,14 +18,13 @@
 
 public class PreallocateTests extends ESTestCase {
     public void testPreallocate() throws IOException {
+        assumeFalse("no preallocate on windows", System.getProperty("os.name").startsWith("Windows"));
         Path file = createTempFile();
         long size = 1024 * 1024; // 1 MB
         var nativeAccess = NativeAccess.instance();
         nativeAccess.tryPreallocate(file, size);
         OptionalLong foundSize = nativeAccess.allocatedSizeInBytes(file);
         assertTrue(foundSize.isPresent());
-        // Note that on Windows the fallback setLength is used. Although that creates a sparse
-        // file on Linux/MacOS, it full allocates the file on Windows
         assertThat(foundSize.getAsLong(), equalTo(size));
     }
 }