feat: implement bb.js browser tests (karma)

[jonybur]

--- This is a draft PR, please don't review ---

[TomAFrench]

Heads up:

Note this will need to be migrated to Aztec packages with the script and merged there. We allow admin bypass for this repo but hope to only use that for release please (and nothing going forward once Aztec has release please)

#695 (comment)

[kevaundray]

Right -- this should be migrated to aztec-packages and also linked to an issue/PR description explaining why we are making this change

[ludamad]

Note for completeness, if you find it useful you can develop it here, let me know if you need some CI turned on, but at the end there's a script in aztec-packages: ./scripts/migrate_barretenberg_branch.sh jb/bbjs-browser-tests-karma "feat: implement..."