AbstractJUnit4BaseTest: Copy getRandomRequestURI() from deprecated
DatabaseBasedTest

- Deprecates the one in DatabaseBasedTest, so copypasting it is fine as
the other one will then be removed someday.
- Will be needed for fixing SubscriptionManagerFCPTest to compile
against AbstractJUnit4BaseTest being the new base class
Abstract*Test: Make the classes really abstract

They were not abstract since JUnit will then complain because it cannot
instantiate them, so they would have to be ignored, but that could not
be done because they contained self-tests.
But I think self-tests are also possible using member classes. So this
commit ...:
- makes them abstract
- adds an @Ignore("") which contains an explanation of how to add self
tests
- removes the cheap self-test at AbstractJUnit4BaseTest as child tests
will explode anyway if it is null, not worth converting a that trivial
one to a member class.
- does not affect any self-test at AbstractFullNodeTest because there is
no self-test in that class anymore already
AbstractJUnit4BaseTest: Copy addRandomIdentities() from deprecated
DatabaseBasedTest

- Deprecates the one in DatabaseBasedTest, so copypasting it is fine as
the other one will then be removed someday due to the deprecation.
- Will be needed for fixing SubscriptionManagerFCPTest to compile
against AbstractJUnit4BaseTest being the new top level base class
instead of DatabaseBasedTest
- Adds an abstract getWebOfTrust() to AbstractJunit4BaseTest since that
class does not generate a WebOfTrust object, it is at the responsibility
of the child classes to create one. This is because child classes will
have different ways of creating one: AbstractFullNodeTest will obtain
the WebOfTrust object by loading the actual JAR into a real node; in
opposite to that the future replacement of DatabaseBasedTest will create
dummy WebOfTrust objects without a node. So it must be up to the child
class how to create a WebOfTrustObject.