My_Test_Repo/force-app/main/default/classes/TestBrevoContactTrigger_Handler.cls

@isTest
public class TestBrevoContactTrigger_Handler {
        @isTest
    static void testBrevoContactTrigger_Handler() {
        // Create a new contact
        Contact con = new Contact(LastName = 'Test', Email = 'test@example.com');
        insert con;
        // Set the mock callout class 
        Test.setMock(HttpCalloutMock.class, new BrevoMockHttpResponseGenerator());
        // Start the test, this enables governor limit testing and prevents actual callouts
        Test.startTest();
        // Call your method here
        BrevoContactTrigger_Handler.createContactInBrevo(con.Email, con.FirstName, con.LastName, con.Phone, con.Id);
        // Stop the test, this changes limit context back to test from trigger
        Test.stopTest();
    }
    @isTest
    static void testUpdateContactInBrevo_ExistingContact() {
        // Create a new contact
        Contact con = new Contact(LastName = 'Test', Email = 'test@example.com');
        insert con;
        // Set the mock callout class 
        Test.setMock(HttpCalloutMock.class, new BrevoMockHttpResponseGenerator());
        // Start the test
        Test.startTest();
        // Call your method here
        BrevoContactTrigger_Handler.updateContactInBrevo(con.Email, con.FirstName, con.LastName, con.Phone, con.Id);
        // Stop the test
        Test.stopTest();
    }
    @isTest
    static void testUpdateContactInBrevo_NonExistingContact() {
        // Create a new contact
        Contact con = new Contact(LastName = 'Test', Email = 'test2@example.com');
        insert con;
        // Set the mock callout class 
        Test.setMock(HttpCalloutMock.class, new BrevoMockHttpResponseGenerator());
        // Start the test
        Test.startTest();
        // Call your method here
        BrevoContactTrigger_Handler.updateContactInBrevo(con.Email, con.FirstName, con.LastName, con.Phone, con.Id);
        // Stop the test
        Test.stopTest();
    }
      @isTest
    static void testDeleteContactInBrevo() {
        // Create a new contact
        Contact con = new Contact(LastName = 'Test', Email = 'test@example.com');
        insert con;
        // Set the mock callout class 
        Test.setMock(HttpCalloutMock.class, new BrevoMockHttpResponseGenerator());
        // Start the test
        Test.startTest();
        // Call your method here
        BrevoContactTrigger_Handler.deleteContactInBrevo(con.Email);
        // Stop the test
        Test.stopTest();
    }
}