jvm-services.gradle.kts

Dogfood native test fixtures
This commit replaces our custom test fixtures with the native "Java test fixtures".
The `TestFixturesPlugin` of our build has been simplified to leverage the native
test fixtures capabilities. Some noticeable changes:

- the `testFixtures` extension has been replaced with regular dependency declaration
- dependencies on test fixtures are now declared using the `testFixtures` keyword
- test fixtures properly declare an API and an implementation, meaning that the
implementation dependencies of test fixtures no longer leak into the compile
classpath of consumers (in particular tests)