Errors on test runs

A failed test run doesn't always mean a test failed. Sometimes a target's .xctest bundle won't load, or the app under test never launches, so the tests never get to run at all. Xcode treats these as errors rather than test failures, and now Tuist does too: run and target-level errors show up in a dedicated Errors section on the test run page, each with the target that hit it and the underlying message.

Pulling these out of your test cases keeps your counts honest, an error is no longer miscounted as a failed test, and makes the two kinds of problem easy to tell apart. We also renamed the failures section to Test Failures, so each reads as what it is: Errors stopped your tests from running, Test Failures are the assertions that didn't hold.