Inline the `uniffi-tests` logic into `tests`
requested to merge SalvatoreT/uniffi-kotlin-multiplatform-bindings:salvatoret/inline-sample-gradle-files into main
Make it easier to copy/paste the plugin logic into new projects. The conventions project cuts down on repeated code, but it makes it harder to see what's going on under the hood.
Use dependencySubstitution in the top level settings so that copying plugin { id("io.gitlab.trixnity.uniffi.kotlin.multiplatform") } is easier too.