how do you ensure them test themselves are correct and 100% accurate
If you mean how to avoid making false positives in test, of course tests like any other code should be peer-reviewed by your teammates. And most importantly, you can check the code coverage for your unit tests so that you know that the tests are actually covering your branches and statements of the code accurately.
As for dealing with codebase that has no tests, I feel you. I've been there before. What was tricky is actually splitting the code so that it's testable, between pure functions and side effects. That should be the first step even before writing any tests. Then, start increasing test coverage gradually, starting with helpers/small pure function modules down to components. FWIW my experience is from a frontend codebase, but I hope you get the gist of it.
Small project, prototypes, yes maybe your argument about not having tests is valid for these. But, as soon as those small projects are used by someone else (maybe you made it open-source), and/or has the possibility to get complex, then that's where tests can be beneficial.
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If you mean how to avoid making false positives in test, of course tests like any other code should be peer-reviewed by your teammates. And most importantly, you can check the code coverage for your unit tests so that you know that the tests are actually covering your branches and statements of the code accurately.
As for dealing with codebase that has no tests, I feel you. I've been there before. What was tricky is actually splitting the code so that it's testable, between pure functions and side effects. That should be the first step even before writing any tests. Then, start increasing test coverage gradually, starting with helpers/small pure function modules down to components. FWIW my experience is from a frontend codebase, but I hope you get the gist of it.
Small project, prototypes, yes maybe your argument about not having tests is valid for these. But, as soon as those small projects are used by someone else (maybe you made it open-source), and/or has the possibility to get complex, then that's where tests can be beneficial.