test: update test to reliably await connection pool minsize

Author: dariakp

test/integration/server-selection/server_selection.prose.operation_count.test.ts

@@ -1,6 +1,5 @@
 import { expect } from 'chai';
-import { setTimeout } from 'timers';
-import { promisify } from 'util';
+import { on } from 'events';
 
 import { CommandStartedEvent } from '../../../src';
 import { Collection } from '../../../src/collection';