Cleanup and improve NatureTest eclipse-platform#903

Simplifies the NatureTest class as preparation to migrate from JUnit 3:
* Removes unnecessary try-catch blocks or replaces them with
assertThrows statements
* Replaces fail() operations called in other threads by passing
exception outside
* Makes test methods rethrow exception rather than calling fail()
* Improves testBug338055() by joining spawned job

Contributes to
eclipse-platform#903