Do not use mock engine in PrimaryAllocationIT (#44083)

PrimaryAllocationIT#testForceStaleReplicaToBePromotedToPrimary 
relies on the flushing when a shard is no long assigned. This behavior,
however, can be randomly disabled in MockInternalEngine.

Closes #44049

server/src/test/java/org/elasticsearch/cluster/routing/PrimaryAllocationIT.java

@@ -91,6 +91,12 @@ protected Collection<Class<? extends Plugin>> nodePlugins() {
         return Arrays.asList(MockTransportService.TestPlugin.class, InternalSettingsPlugin.class);
     }
 
+    @Override
+    protected boolean addMockInternalEngine() {
+        // testForceStaleReplicaToBePromotedToPrimary replies on the flushing when a shard is no longer assigned.
+        return false;
+    }
+
     public void testBulkWeirdScenario() throws Exception {
         String master = internalCluster().startMasterOnlyNode(Settings.EMPTY);
         internalCluster().startDataOnlyNodes(2);