Add unit test for AttentionBank::getRandomAtomNotInAF()
Add unit test for AttentionBank::getRandomAtomNotInAF()

Previous AttentionBank::getRandomAtomNotInAF() implementation
expects that it can get some random atom from _importanceIndex
which is out of attentionalFocus collection in 50 attempts. Add unit
test which demonstrates that it is not always true.

rTreutlein/atomspace,AmeBel/atomspace,rTreutlein/atomspace,AmeBel/atomspace,AmeBel/atomspace,rTreutlein/atomspace,AmeBel/atomspace,AmeBel/atomspace,rTreutlein/atomspace,rTreutlein/atomspace