AbstractJUnit4BaseTest.addRandomTrustValues(): Validate parameters

- The requirement of having less than identities² trust values in the
JavaDoc is converted to an assert
- A Java assert() is used since callers can be assumed to be not that
stupid. Unit tests will hava Java asserts enabled the most time anyway
probably.
AbstractJUnit4baseTest.addRandomTrustValues(): Fix bogus assert()

Identitys can only trust other Identitys than themselves. So for each of
the N identities, it can trust N-1.
AbstractJUnit4BaseTest: Javadoc @throws

In unit tests, I usually add all expections which callees declare to the
throws declaration of the caller so they are thrown out to JUnit and
detected as test failure. So we can add this to the JavaDoc.