[xtend][tests] Fixed bogus test cases, added negative test

Follow-up for https://bugs.eclipse.org/bugs/show_bug.cgi?id=452602

Change-Id: I93b5a640c82cecf46f34a893c1f003f310832961
[xtext][document] Regression: Fixed NPE in unit test

see https://bugs.eclipse.org/bugs/show_bug.cgi?id=452515

Change-Id: I50d46308918fc1170846de68567a9d5da0d95dac
Merge "[xtend][tests] Fixed bogus test cases, added negative test"
Merge "[xtext][document] Regression: Fixed NPE in unit test"