Fix unit test classpath when testing shadow jar projects

[mark-vieira]

The pull request sorts out a configuration ordering issue when modifying unit test task classpaths to test against a shadow jar instead of the normal classes directory. Instead of trying to mutate the existing task's classpath, we use the test sourceset runtime classpath isntead. We also limit this to only apply to unit test (i.e. the test task).

[elasticmachine]

Pinging @elastic/es-delivery (Team:Delivery)

[mark-vieira]

@elasticmachine update branch

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
❌	7.14	Error pushing to https://github.com/elasticsearchmachine/elasticsearch. Repository does not exist. Either fork the source repository (https://github.com/elastic/elasticsearch) or disable fork mode "--fork false". Read more about "fork mode" in the docs: https://github.com/sqren/backport/blob/3a182b17e0e7237c12915895aea9d71f49eb2886/docs/configuration.md#fork
❌	7.x	Error pushing to https://github.com/elasticsearchmachine/elasticsearch. Repository does not exist. Either fork the source repository (https://github.com/elastic/elasticsearch) or disable fork mode "--fork false". Read more about "fork mode" in the docs: https://github.com/sqren/backport/blob/3a182b17e0e7237c12915895aea9d71f49eb2886/docs/configuration.md#fork

To backport manually run:
backport --pr 74783