Begin using Swift Testing for unit tests

[briancroom]

With Swift Testing now fully-integrated into Swift toolchains since the 6.0 release, Swift Driver should start using in its test suite, both for new tests, as well as incrementally migrating existing XCTest tests.

This issue specifically tracks the step of introducing a first Swift Testing test file into the project (either a new test, or migrating an existing one), including working through any CI/configuration/infrastructure adjustments that may be required for this to happen.