Add test for IndexFormatTooOldException if a commit has a 3.x segment [LUCENE-6213]

We should add a 4.x index (4.x commit) with some 3.x segment(s) to our backwards tests.

I don't think we throw IndexFormatTooOldException correctly in this case. I think instead the user will get a confusing SPI error about a missing codec "Lucene3x".

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Migrated from [LUCENE-6213](https://issues.apache.org/jira/browse/LUCENE-6213) by Robert Muir (@rmuir), updated Apr 29 2015
Attachments: [LUCENE-6213.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-6213/LUCENE-6213.patch) (versions: 3), [unsupported.4x-with-3x-segments.zip](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-6213/unsupported.4x-with-3x-segments.zip)

Start with Lucene's backwards-compatibility tests and the attached unsupported.4x-with-3x-segments.zip; reproduce opening the 4.x commit containing 3.x segments. Verify the failure is reported as IndexFormatTooOldException rather than a missing "Lucene3x" codec, then add the regression test.

Written by the indexing model from the issue text.