Reenable+Fix testMasterShutdownDuringFailedSnapshot (#46303) (#46747)

Reenable this test since it was fixed by #45689 in production
code (specifically, the fact that we write the `snap-` blobs
without overwrite checks now).
Only required adding the assumed blocking on index file writes
to test code to properly work again.

* Closes #25281

server/src/test/java/org/elasticsearch/snapshots/DedicatedClusterSnapshotRestoreIT.java

@@ -933,7 +933,6 @@ public void testMasterAndDataShutdownDuringSnapshot() throws Exception {
         }
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/25281")
     public void testMasterShutdownDuringFailedSnapshot() throws Exception {
         logger.info("-->  starting two master nodes and two data nodes");
         internalCluster().startMasterOnlyNodes(2);

test/framework/src/main/java/org/elasticsearch/snapshots/mockstore/MockRepository.java

@@ -369,6 +369,9 @@ public void writeBlob(String blobName, InputStream inputStream, long blobSize, b
             public void writeBlobAtomic(final String blobName, final InputStream inputStream, final long blobSize,
                                         final boolean failIfAlreadyExists) throws IOException {
                 final Random random = RandomizedContext.current().getRandom();
+                if (blobName.startsWith("index-") && blockOnWriteIndexFile) {
+                    blockExecutionAndFail(blobName);
+                }
                 if ((delegate() instanceof FsBlobContainer) && (random.nextBoolean())) {
                     // Simulate a failure between the write and move operation in FsBlobContainer
                     final String tempBlobName = FsBlobContainer.tempBlobName(blobName);