Do it anyway. The thing about testing is that it's a skill that you have to work on. You have to know what to test and how. Testing shouldn't affect the speed at which you write code at all.
The reason your boss is saying no is because you had to ask him. The reason you had to ask them is because you know it will take more time than it will save, at least at first.
You have to learn this skill somehow, and the best way to do it is on something that matters rather than with a side project. So write tests at your job, and learn the skill of testing on your employer's time, without their knowledge. Or on off hours if that makes you squeamish. But learn the skill, it's important.
Then, when you refine your testing work flow to the point where it makes more sense to test as you write, don't bother hiding it anymore. When they ask, show them your workflow and how it's not taking up too much time and list out the benefits of testing. If they tell you to stop anyway, take your new skills and find a new job. You're growing past the ability of your current job to challenge you.
The reason your boss is saying no is because you had to ask him. The reason you had to ask them is because you know it will take more time than it will save, at least at first.
You have to learn this skill somehow, and the best way to do it is on something that matters rather than with a side project. So write tests at your job, and learn the skill of testing on your employer's time, without their knowledge. Or on off hours if that makes you squeamish. But learn the skill, it's important.
Then, when you refine your testing work flow to the point where it makes more sense to test as you write, don't bother hiding it anymore. When they ask, show them your workflow and how it's not taking up too much time and list out the benefits of testing. If they tell you to stop anyway, take your new skills and find a new job. You're growing past the ability of your current job to challenge you.