Simplify the unit test build step

[kzar]

Remove the buildTests script, since we can bundle the unit test files
with Browserify directly from the Makefile. In the future, we should
remove shared/content-scope-scripts entirely, but making that work
with Browserify is proving to be tricky.

Reviewer: @sammacbeth

Automated tests:

• Unit tests
• Integration tests

Reviewer Checklist:

• Ensure the PR solves the problem
• Review every line of code
• Ensure the PR does no harm by testing the changes thoroughly
• Get help if you're uncomfortable with any of the above!
• Determine if there are any quick wins that improve the implementation

PR Author Checklist:

• Get advice or leverage existing code
• Agree on technical approach with reviewer (if the changes are nuanced)
• Ensure that there is a testing strategy (and documented non-automated tests)
• Ensure there is a documented monitoring strategy (if necessary)
• Consider systems implications