Writing good tests means you don't have to find and read the 200 relevant lines of code.
The quality of the code is a combination of the quality of the programmers and the quality of the processes they use to produce the code. If you have really good programmers, they can use a shoddy process (write, compile, ship) and still produce reasonable quality code; if you have average programmers with a good process (write test, write code, compile, fix warnings, compile, run test suite, fix errors, compile, run test suite, ship), you can produce good code.
What I've found is that whether a test will be useful or not depends on the nature of the code. If I'm writing logic code or building a domain-specific language, an automated test suite means the difference between getting it right quickly and taking a long time to get it hopefully probably mostly right.
Readability is only one aspect of code quality, and code that does something complex and convoluted will never be simple and straightforward. For that matter, complex, convoluted, and correct is preferable to simple, straightforward, and wrong, and that's the sort of thing that a test suite will tell you.
Readability and correctness are orthogonal concepts.