Xcode has a built-in breakpoint that breaks on test failures. When this breakpoint is set, the execution of the tests is stopped, and a debug session is started whenever a test fails.
Usually, this is not what you want because in TDD failing tests are normal, and you don't need a debugger to find out what's going on. You explicitly wrote the test to fail at the beginning of the TDD workflow cycle.
But in case you need to debug one or more failing tests, it's good to know how this breakpoint is activated. Open the Debug Navigator:
At the bottom of the navigator view is a button with a plus sign (+). Click ...