How to reproduce:
1. Open the "AssertError" project
2. Open the Test Runner
3. Run tests, "Test" fails

Expected result: "Test" test should pass.

Reproducible with: 2017.3.0a2, 2017.2.0b5, 2017.1.0p1, 5.6.3f1, 5.6.1f1.

Note: "Test2" is the same script as "Test" just without frame skipping (It passes).

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This is by design. One can only LogAssert on a message that has been just logged if you do it in the same frame. If you skip frames, you have to LogAssert before the actual call generating the log. So, the test rewritten like this works:

[UnityTest]
public IEnumerator TestDebugLog()
{
LogAssert.Expect(LogType.Error, "Testerror");
yield return null; yield return null;
Debug.LogError("Testerror");
}