add test to check if ros node is loadable

If we use python2 PYTHON_INTERPRETER on 20.04, python2 fails to load rospy in /opt/ros/noetic, because rospy moduels are alraedy updated.
If we use python3 PYTHON_INTERPRETER on 18.04, python3 can load rospy in /opt/ros/melodic by chance.

c.f. #367

ros_google_cloud_language/CMakeLists.txt

@@ -52,7 +52,7 @@ catkin_generate_virtualenv(
 
 include_directories()
 
-file(GLOB NODE_SCRIPTS_FILES node_scripts/*.py)
+file(GLOB NODE_SCRIPTS_FILES node_scripts/*.py test/*.py)
 file(GLOB SCRIPTS_FILES scripts/*.py)
 catkin_install_python(
   PROGRAMS ${NODE_SCRIPTS_FILES} ${SCRIPTS_FILES}
@@ -68,3 +68,7 @@ install(FILES requirements.txt
   DESTINATION ${CATKIN_PACKAGE_SHARE_DESTINATION}
 )
 
+if(CATKIN_ENABLE_TESTING)
+  find_package(rostest REQUIRED)
+  add_rostest(test/test_rospy_node.test)
+endif()

ros_google_cloud_language/test/test_rospy_node.py

@@ -0,0 +1,20 @@
+#!/usr/bin/env python
+
+import rospy
+import os, sys, unittest, rostest
+
+class TestRospyNode(unittest.TestCase):
+
+    def __init__(self, *args):
+        unittest.TestCase.__init__(self, *args)
+
+    def test_rosnode(self):
+        __name__ = 'dummy'
+        scripts_dir = os.path.abspath(os.path.join(os.path.realpath(__file__), os.pardir, os.pardir, 'node_scripts'))
+        for filename in [f for f in map(lambda x: os.path.join(scripts_dir, x), os.listdir(scripts_dir)) if os.path.isfile(f)]:
+            print("Check if {} is loadable".format(filename))
+            exec(open(filename).read()) in globals(), locals()
+            self.assertTrue(True)
+
+if __name__ == '__main__':
+    rostest.rosrun('test_rospy_node', 'ros_google_cloud_language', TestRospyNode, sys.argv)

ros_google_cloud_language/test/test_rospy_node.test

@@ -0,0 +1,3 @@
+<launch>
+  <test pkg="ros_google_cloud_language" type="test_rospy_node.py" test-name="ros_rospy_node" />
+</launch>