When JUnit tests have assumption failures, treat them as "skipped"

JUnit has a conception of "assumptions", when encode conditions in which a test is meaningful.
When an assumption fails, it means that the test provides no useful information, and should
"halt and be ignored".  Prior to this change, Gradle was treating JUnit tests with assumption
failures as "passed".

http://junit.sourceforge.net/javadoc/org/junit/Assume.html