Add unit test for chaining proxy

During creation of some python tests it turned out that code that limits length of chain of proxy accounts has some holes in it. The test uses the hole to create chain that is too long according to rules. It also checks that propagation of voting power stops on HIVE_MAX_PROXY_RECURSION_DEPTH (4), so there is no threat from too long chain (we could create arbitrarily long chain and the effect would be that all but 4 top accounts don't influence governance vote power, there would be no excessive execution time).