Replace noPassthrough/indexbg2.js with a test that uses failpoints. This test will confirm expected behavior of duplicate key insertion during background unique index builds. Included will be checks that unique background index build:

1. Fails when there exists duplicate indexed values for the duration of the build.
2. Fails when started with a unique key set, but a document with a duplicate key is inserted prior to that key being indexed.
3. Succeeds when a document is inserted with a key that has already been indexed (with the insert failing on duplicate key error).
4. Succeeds when a document with a key not present in the initial set is inserted twice (with the initial insert succeeding and the second failing on duplicate key error).