Begin using Swift Testing for unit tests

[briancroom]

With Swift Testing now fully-integrated into Swift toolchains since the 6.0 release, swiftly should start using it in its test suite, both for new tests, as well as incrementally migrating existing XCTest tests. This is aligned with the project's goals of using modern Swift APIs and idioms.

[briancroom]

The new Test Scoping Traits feature in Swift 6.1 may be useful here when migrating the existing test code which is utilizing an XCTest class-level teardown method.

[cmcgee1024]

See #304

[cmcgee1024]

Everything is migrated over now on the main branch.