I am a senior computer science student. I will graduate this summer, but my knowledge and projects are little. I am also a good programmer, but I didn't take any certificates or interns or anything to fill my CV with. So I was thinking to make myself fail this year, and start senior year again just to take interns and have more experience in my field and take more courses. So my question is does the employer care if I fail a year? And do you think I should do this or not?
Making yourself fail to earn more experience is a bad decision. There are many graduate with no extra certificates or intensive work experience just like you.
Instead, I would recommend you to build a portfolio for yourself. Start developing some POC application with your desired programming language and place them in communities like github and make a reference to them in your CV. Create something that worth mentioning and displaying during your job interview.
If you have no idea what to implement, or don't know where to start. Try reading books that teaches programming by implementing one application phase by phase. (there are plenty of books like this)
Do not fail a year! That will not be good for your GPA and while some employers don't much care about it, it can be important to getting your first job after university. Employers notice if it takes someone n+1 years to finish an n-year program and it doesn't look good on a resume.
Having internships on your resume is important, but I don't think getting just a couple (I'm not sure how many you can do in your final year) is worth the hit to your GPA and making employers wonder why you needed an extra year to finish your degree.
I think it would be better for you in the long run to graduate on time, potentially take a job that isn't exactly your dream job, and improve your resume by building your own projects and/or contributing to open source in your free time.