SOLR-5118: beef up assertions in SolrIndexConfigTest, this allows us to remove TestPropInject and TestPropInjectDefaults -- neither of which were testing property injection (tested elsewhere) but were doing some basic assertions on the parsing & init of merge policy and merge scheduler (merge r1510830)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1510833 13f79535-47bb-0310-9956-ffa450edef68