Write the Test Suite

@isTest
private class MilestoneTriggerTest {
    @isTest
    static void allowsValidStatusTransitionsInBulk() {
        Project__c proj = new Project__c(Name = 'Test Project', Status__c = 'Active');
        insert proj;

        List<Milestone__c> milestones = new List<Milestone__c>();
        for (Integer i = 0; i < 200; i++) {
            milestones.add(new Milestone__c(Project__c = proj.Id, Status__c = 'Not Started'));
        }
        insert milestones;

        for (Milestone__c m : milestones) m.Status__c = 'In Progress';

        Test.startTest();
        update milestones;
        Test.stopTest();

        for (Milestone__c m : [SELECT Status__c FROM Milestone__c WHERE Project__c = :proj.Id]) {
            Assert.areEqual('In Progress', m.Status__c);
        }
    }
}

200 Milestones updated in one bulk operation — Module 31\'s "always test at bulk scale" rule, still the very first check for any trigger-bearing object.

@isTest
static void marksProjectAtRiskWhenMilestoneOverdue() {
    Id fakeProjectId = TestHelper.fakeId(Project__c.SObjectType);
    Milestone__c overdue = new Milestone__c(
        Id = TestHelper.fakeId(Milestone__c.SObjectType),
        Project__c = fakeProjectId,
        Status__c = 'In Progress',
        Due_Date__c = Date.today().addDays(-3)
    );

    FakeMilestoneSelector fakeSelector = new FakeMilestoneSelector(new List<Milestone__c>{overdue});
    ProjectHealthService service = new ProjectHealthService(fakeSelector);

    Map<Id, String> result = service.calculateHealth(new Set<Id>{fakeProjectId});

    Assert.areEqual('At Risk', result.get(fakeProjectId));
}

This is Lesson 5\'s dependency injection paying off directly in the test suite — ProjectHealthService\'s core business rule is verified without inserting a single real record, running dramatically faster than a DML-based test would.

@isTest
static void recalculatesHealthForAllActiveProjects() {
    // ... insert an Active Project with an overdue Milestone

    Test.startTest();
    Database.executeBatch(new RecalculateProjectHealthBatch());
    Test.stopTest(); // forces the batch to actually run

    // assert the Project's Health__c field was updated correctly
}

Test.startTest()/Test.stopTest() forcing the batch to run synchronously within the test — Module 31\'s and Module 38\'s exact async-testing pattern, reused one final time for this project\'s nightly safety net.