Build and test in CI using `swift` (in addition to `xcodebuild`)

[bachand]

Inspired by https://github.com/dfed/CacheAdvance/pull/42#discussion_r415180088.

Goals

• Ensure that changes to this library don't break building and testing with swift directly.

Non-Goals

• Test all OS configurations using swift. We can continue to have our comprehensive test matrix use xcodebuild.
• Remove tests that rely on xcodebuild in favor of swift. ROI is not there, and its nice to ensure that the generated project works.

Investigations n/a

Design

Build and test via swift in CI to sanity to check that this continues to work.