Unit tests for duplicate check

Add unit tests to ensure we can:

- separately search for exact-duplicates without searching for near-duplicates
- separately search for near-duplicates without searching for exact-duplicates
- search for both in single call to imagelab.find_issues()
- search for exact duplicates in initial call to imagelab.find_issues() and then search for near duplicates in subsequent call to this same function from the same imagelab
- search for near duplicates in initial call to imagelab.find_issues() and then search for exact duplicates in subsequent call to this same function from the same imagelab

Start at imagelab.find_issues() and trace how exact-duplicate and near-duplicate searches are selected and retained between calls. Add unit tests covering the five listed call sequences, including separate searches, combined searches, and reversed successive searches on the same imagelab instance.

Written by the indexing model from the issue text.