Fix testBlobStoreCache (elastic#79747)

The attempt to fix this test in elastic#78616 wasn't complete: we need to wait
for all mounted indices, not only the last restored one, before deleting
the indices. Otherwise one of the mounted index might still be
initializing and recreates the cached documents in
`.snapshot-blob-cache`  Closes elastic#78993

x-pack/plugin/searchable-snapshots/src/internalClusterTest/java/org/elasticsearch/xpack/searchablesnapshots/cache/blob/SearchableSnapshotsBlobStoreCacheIntegTests.java

@@ -304,7 +304,8 @@ public Settings onNodeStopped(String nodeName) throws Exception {
                     .build();
             }
         });
-        ensureGreen(restoredAgainIndex);
+
+        ensureGreen("restored-*");
 
         assertRecoveryStats(restoredAgainIndex, false);
         assertExecutorIsIdle(SearchableSnapshots.CACHE_FETCH_ASYNC_THREAD_POOL_NAME);