Fix documentation of custom tasks

[quijote]

Adapts the docs at https://gradleup.com/shadow/custom-tasks/.

Using testImplementation does not work anymore with latest Gradle versions. It produces the following error:

Could not determine the dependencies of task ':testJar'.
> Resolving dependency configuration 'testImplementation' is not allowed as it is defined as 'canBeResolved=false'.
  Instead, a resolvable ('canBeResolved=true') dependency configuration that extends 'testImplementation' should be resolved.

Instead, Gradle docs hints that testRuntimeClasspath is a resolvable configuration.