Fix replacement example inference tree expansion

[ngeiswei]

Problem

Some BC inference tree expansion fails. The problem can be reproduced by running

https://github.com/opencog/ure/tree/master/examples/ure/replacement

As one can see from the log, in iteration 3, where the case (synonymous (Predicate "believe") (Variable "$B")) is considered as unification, the subsequent inference tree expansion fails. The unification is correct, so the problem comes from the expansion itself.

Tasks

• Create unit test based on that example in BITUTest.cxxtest, named test_expand_4 or such.
• Fix BIT expansion to pass the unit test.