having dirty tests is equivalent to, if not worse than, having no tests. The problem is that tests must change as the production code evolves. The dirtier the tests, the harder they are to change. The more tangled the test code, the more likely it is that you will spend more time cramming new tests into the suite than it takes to write the new production code
Well-written and outstanding unit test increase the quality of project. Basically, the general purpose or aim of a project is to provide resolution with meaningful change that are able to resolve some issues. With clean test unit, the project may be able to evolve to satisfy its ultimate goal.
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