CI: run tests

[Liampronan]

Following up from this PR: #23 – I ran into issues getting the tests to run in CI despite running locally. So, we need to figure out how to get tests to run.

• run tests
• add dynamic build badge to readme
• consider if caching deps makes sense for build process

The issues I ran into

• using swift build test - the tests can't ran because they rely on UIKIt (UIImage).
• using xcodebuild test - there were issues finding the right scheme

What's confusing to me:

• the tests work in Xcode when I open Package.swift and hit cmd+u.

[Liampronan]

some thoughts on next steps from today's attempt -- again fundamental issue seems to be mismatch between swift package tests running in xcode vs. workspace