Ideally, we should write bug-free code, but we can't. There are some tools to avoid bugs, tests are one of them. Those tools avoid them, but not mitigate. Bisect doesn't replace tests, it only helps find where the bugs are happening. After finding and fixing the bugs, it's a good idea to write a test covering that bug.

To sum up: bisect and tests are not in opposite sides, they complement each other