I'm very, very new to Apex and at a loss. I have a business requirement to update a custom Lead picklist field called "Qualification Status" to "Converted" when a Lead is converted. I've created the following Trigger:

trigger LeadQualStatusToConvertedTrigger on Lead (before update) {
    
    List<Lead> convertedLeads = new List<Lead>();
    
    for(Lead convertedLead : Trigger.New) {
        if (convertedLead.IsConverted) {
            convertedLeads.add(convertedLead);
        }
    }

    LeadQualStatusToConverted.setQualStatusConverted(convertedLeads);
}

I've tested LeadQualStatusToConverted.setQualStatusConverted() and it works. The issue is that nothing satisfies the Trigger's If clause. Here's a relevant piece of my test class:

@testSetup
    static void setup() {
        List<Lead> testLeads = new List<Lead>();
        for(Integer i=0; i<10; i++) {
            testLeads.add(new Lead(LastName = 'Smith' + i
                                  ,Company = 'Smith Co.'
                                  ,Qualification_Status__c = 'MQL'
                                  ,IsConverted = False));
        }
        
        Database.insert(testLeads);
    }

    @isTest
    static void TestUpdatedConvertedLead() {
        Lead testLead = [Select Id, Qualification_Status__c, IsConverted
                         From Lead
                         Where LastName='Smith1'];

        Test.startTest();
        Database.LeadConvert lc = new Database.LeadConvert();
        lc.setLeadId(testLead.Id);
        lc.setConvertedStatus('Qualified');
        Database.LeadConvertResult lcr = Database.convertLead(lc);
		Test.stopTest();
        
        System.assert(True,testLead.IsConverted);
        System.assert(lcr.isSuccess());
        System.assertEquals('Converted',testLead.Qualification_Status__c);
    }

The first two asserts pass, but the one that matters (the third), does not. The test result statest that the Qualification_Status__c is still the initialized value of MQL. So, it's successfully being converted, but not making it past the Trigger's If and thus not being passed to the setQualStatusConverted() method.

I'm at a loss... How can I check for IsConverted in the Trigger?