Remove test dependency on Spock
Remove test dependency on Spock

Spock version 0.6-groovy-1.8 is incompatible with Groovy 2.4; however,
Spock is currently not even used within the Gradle plugin project. This
commit therefore removes the test dependency on Spock.

sbrannen/junit-lambda,marcphilipp/junit5,junit-team/junit-lambda,marcphilipp/junit-lambda