Showing that the Apex Flex Queue does not appear to work in the context of Unit Tests.

FlexQueueExample.cls

global class FlexQueueExample implements Database.Batchable<sObject>{

    global Iterable<sObject> start(Database.BatchableContext BC) { 
        return new List<sObject> {};
    }
    
    global void execute(Database.BatchableContext BC, List<sObject> scope){}
    global void finish(Database.BatchableContext BC){}
    
    // Works in Execute Anonymous for Count <= 100
    global static void run(Integer count){
        for (integer i=0; i<count; i++){
            database.executeBatch(new FlexQueueExample());
        }
    }
}

FlexQueueExampleTest.cls

@isTest (seeAllData = false)
private class FlexQueueExampleTest {

    //System.LimitException: Attempted to schedule too many concurrent batch jobs in this org (limit is 5).
    private static testMethod void shouldBeAbleToUseFlexQueueInTests(){
        FlexQueueExample.run(10);
    }

}