Avoid copying buidSrc/build directory for running integration tests

[alexander-yevsyukov]

The class io.spine.tools.gradle.testing.GradleProject which helps with initializing Gradle projects in tests has private writeBuildSrc() method which copies the content of the buildSrc folder from the root of the project into a temp directory created for an integration test.

The current implementation copies not only source code, but also the build directory. If the build directory is not copied, integration tests fail because Groovy-based Gradle scripts cannot resolve Kotlin-based dependencies (declared in buildSrc/src/main/kotlin/io/spine/internal/dependency).

It is still not clear to me why the Kotlin-based dependencies work for a clean project and do not work in tests, but I don't think we should invest too much time into clarifying this “mystery”. I would put time and efforts into migrating build scripts from Groovy to Kotlin instead.