feat: implement bb.js browser tests (web-test-runner)

[jonybur]

-WORK IN PROGRESS; DO NOT REVIEW-

Description

Please provide a paragraph or two giving a summary of the change, including relevant motivation and context.

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• The branch has been merged with/rebased against the head of its merge target.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• There are no circuit changes, OR a cryptographer has been assigned for review.
• New functions, classes, etc. have been documented according to the doxygen comment format. Classes and structs must have @brief describing the intended functionality.
• If existing code has been modified, such documentation has been added or updated.
• No superfluous include directives have been added.
• I have linked to any issue(s) it resolves.
• I'm happy for the PR to be merged at the reviewer's next convenience.

[kevaundray]

This should be moved to aztec-packages as noted in the #696 -- just looking at the PR titles, I can't tell what the difference/relevance is between this and #696, so a PR description would be good