ff: add unit tests for BitIterator

[tcoratger]

Description

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• Targeted PR against correct branch (master)
• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the GitHub PR explorer

[Pratyush]

thank you! a randomized fuzz-style test might be even better: generating random u64s, bit decomposing them, and checking that the output of BitIterator matches.