Don't shutdown server until we're done with the starting phase

src/core/server/integration_tests/saved_objects/migrations/group3/incompatible_cluster_routing_allocation.test.ts

@@ -121,7 +121,7 @@ describe('incompatible_cluster_routing_allocation', () => {
     await root.preboot();
     await root.setup();
 
-    root.start().catch(() => {
+    const startPromise = root.start().catch(() => {
       // Silent catch because the test might be done and call shutdown before starting is completed, causing unwanted thrown errors.
     });
 
@@ -165,6 +165,7 @@ describe('incompatible_cluster_routing_allocation', () => {
       { retryAttempts: 100, retryDelayMs: 500 }
     );
 
+    await startPromise; // Wait for start phase to complete before shutting down
     await root.shutdown();
   });
 });