SOLR-6374: Improved TestIntervalFaceting: Avoid asserting on number of segments as that depends on the seed. Skip tests when multiple segments can't be generated easily

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1617863 13f79535-47bb-0310-9956-ffa450edef68