test with matrix of swift versions

[pgautier404]

We're currently using a swift 5.8 container for testing, as there are issues with the standard gh action that provides the swift binary. We are ostensibly (arbitrarily) supporting 5.4 as our minimum, version, so we should include that version in our testing as well (at least). I tried just bumping down the container version, but the tests failed because that version of the language apparently uses a different schema for Package.resolved than 5.8 does. It may just be best to not ship that file so we can test multiple versions, but for right now I just went with 5.8 as the single testing platform. As we explore this issue, it would be nice to see if there's another alternative to the broken swift action than using a container, as the container is very slow and therefore not ideal for testing a matrix of versions.