Add in a test dependency on pytest.

package.xml

@@ -24,6 +24,8 @@
   <exec_depend>rqt_gui_py</exec_depend>
   <exec_depend>rqt_py_common</exec_depend>
 
+  <test_depend>python3-pytest</test_depend>
+
   <export>
     <rqt_gui plugin="${prefix}/plugin.xml"/>
     <build_type>ament_python</build_type>

setup.py

@@ -31,6 +31,7 @@
         'A Python GUI plugin publishing ROS messages.'
     ),
     license='BSD',
+    tests_require=['pytest'],
     entry_points={
         'console_scripts': [
             'rqt_publisher = ' + package_name + '.main:main',