test: Move fosgen test into a step where java is available

.drone.yml

@@ -41,7 +41,6 @@ steps:
       - cd ../bin
       - ln -s ../nailgun/nailgun-client/target/ng .
       - cd ..
-      - bin/fosgen -d tests/emit_sum.fos
   - name: extract_tests
     image: xonsh/xonsh
     commands:
@@ -53,7 +52,8 @@ steps:
         path: /drone/lib
       - name: m2
         path: /root/.m2
-    commands:
+    commands:  # Note we first make sure that fosgen is working
+      - bin/fosgen -d tests/emit_sum.fos
       - bin/generate_test_code
   - name: python_tests
     image: python:slim