Unit tests for duplicate check

[sanjanag]

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