Fix PrefetchVerbTests: clear cache before each test

The multi-entry prefetch cache persists across ordered tests, causing
cache hits where the tests expect fresh prefetch work. Delete
BlobPrefetchCache.dat in [SetUp] so each test starts with a clean cache.

Assisted-by: Claude Opus 4.6
Signed-off-by: Tyrie Vella <tyrielv@gmail.com>

Author: tyrielv

GVFS/GVFS.FunctionalTests/Tests/EnlistmentPerFixture/PrefetchVerbTests.cs

@@ -37,6 +37,16 @@ public PrefetchVerbTests()
             this.fileSystem = new SystemIORunner();
         }
 
+        [SetUp]
+        public void DeletePrefetchCache()
+        {
+            string cachePath = Path.Combine(this.Enlistment.DotGVFSRoot, "BlobPrefetchCache.dat");
+            if (File.Exists(cachePath))
+            {
+                File.Delete(cachePath);
+            }
+        }
+
         [TestCase, Order(1)]
         public void PrefetchAllMustBeExplicit()
         {