Should BaseTokenStreamTestCase catch analyzers that create duplicate tokens? [LUCENE-7622]

The change to BTSTC is quite simple, to catch any case where the same term text spans from the same position with the same position length. Such duplicate tokens are silly to add to the index, or to search at search time.

Yet, this change produced many failures, and I looked briefly at them, and they are cases that I think are actually OK, e.g. `PatternCaptureGroupTokenFilter` capturing (..)(..) on the string `ktkt` will create a duplicate token.

Other cases looked more dubious, e.g. `WordDelimiterFilter`.

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Migrated from [LUCENE-7622](https://issues.apache.org/jira/browse/LUCENE-7622) by Michael McCandless (@mikemccand)
Attachments: [LUCENE-7622.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-7622/LUCENE-7622.patch)

Start with BaseTokenStreamTestCase and the proposed duplicate-token check described in the issue. Reproduce and inspect failures involving PatternCaptureGroupTokenFilter and WordDelimiterFilter, using the attached LUCENE-7622.patch as context. Done means the test distinguishes acceptable duplicate tokens from cases that should be rejected.

Written by the indexing model from the issue text.