Set stub project group

Author: alexander-yevsyukov

gradle-plugins/src/test/java/io/spine/tools/core/jvm/gradle/given/StubProject.java

@@ -75,6 +75,7 @@ public static Project createAt(File projectDir) {
         var project = ProjectBuilder.builder()
                 .withProjectDir(projectDir)
                 .build();
+        project.setGroup("io.spine.stub");
         return project;
     }