Scala 2.13 in gradle causes test failures

[ignasi35]

After #29, #31 and #32 I can no longer use ./gradlew build on a project built with this template accepting defaults:

$ ./gradlew build                                                                                 

> Task :compileJava
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.

> Task :compileTestJava
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.

> Task :test

com.example.helloworld.GreeterTest > classMethod FAILED
    java.lang.NoClassDefFoundError at GreeterTest.java:32
        Caused by: java.lang.ClassNotFoundException at GreeterTest.java:32

com.example.helloworld.GreeterTest > classMethod FAILED
    java.lang.NullPointerException at GreeterTest.java:50

com.example.helloworld.GreeterServiceImplTest > classMethod FAILED
    java.lang.NoClassDefFoundError at GreeterServiceImplTest.java:24

com.example.helloworld.GreeterServiceImplTest > classMethod FAILED
    java.lang.NullPointerException at GreeterServiceImplTest.java:31

4 tests completed, 4 failed

> Task :test FAILED

FAILURE: Build failed with an exception.
...

BUILD FAILED in 13s

[raboof]

Oops