A well-designed type system can replace micro-scale "fast tests" that are basically intended to check that a unit work correctly on all of the data types that it's specified to work on.
The slow tests, that run the entire application, aren't affected. Look at the incredible amount of test cases that the Rust compiler and Servo have that don't even use the normal unit testing framework, because they want to be able to launch the entire user interface and check its inputs and outputs (or, in Servo's case, because the test harness is supposed to work with software that isn't even written in Rust). In my (admittedly limited) experience, those are the tests that eat most of your time, because they end up hitting race conditions and bugs in the test code itself, even if you write less of them and more of the decent unit tests. But they do catch actual bugs that unit tests wouldn't catch.
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