AbstractJUnit4BaseTest: Add getRandomInsertURI()

- This will be needed for moving more code from DatabaseBasedTest to
AbstractJUnit4BaseTest.
- It deprecates the DatabaseBasedTest.getRandomSSKPair(). We don't have
to generate a full keypair anymore since OwnIdentity creation nowadays
only requires an insert URI, not a request URI. The request URI will be
computed from the insert URI.