SOLR-6755: Fix the test to always return 2 parsedqueries i.e. have more 2 shards (merge from trunk)

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1640417 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-6063: allow overriding whether/how ConcurrentMergeScheduler stalls incoming threads when merges are falling behind

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1640457 13f79535-47bb-0310-9956-ffa450edef68