improve assume message in test

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1642823 13f79535-47bb-0310-9956-ffa450edef68
VerboseFS shouldn't suppress exceptions if it doesn't want to log them

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1642826 13f79535-47bb-0310-9956-ffa450edef68
don't allow O(N^2) merging in this test

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_5x@1642828 13f79535-47bb-0310-9956-ffa450edef68