Re-enable tests

[jbmorley]

Unfortunately I had to disable tests as a result of (what I think is) a bug in SwiftPM not correctly handling LuaSwift's generation of both a library and a build plugin. It seems important to re-enable these.

[jbmorley]

See #298 for more context.

[jbmorley]

@tomsci thinks the issue is swiftlang/swift-package-manager#8436, which is theoretically fixed in 6.2.