LUCENE-5902: Handle different location for backcompat tests in trunk vs branch_4x

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1622604 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5902: Fix bug in previous commit

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1622606 13f79535-47bb-0310-9956-ffa450edef68
LUCENE-5902: Correct module name for backcompat codecs

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1622609 13f79535-47bb-0310-9956-ffa450edef68
Add 4.10 codec backcompat test indexes

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1622610 13f79535-47bb-0310-9956-ffa450edef68