Add a test dependency on pytest. (#94)

While there aren't currently tests defined here, this
sets it up so that it is ready for pytest tests.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>

package.xml

@@ -28,6 +28,8 @@
   <exec_depend>rqt_py_common</exec_depend>
   <exec_depend>std_msgs</exec_depend>
 
+  <test_depend>python3-pytest</test_depend>
+
   <export>
     <architecture_independent/>
     <build_type>ament_python</build_type>

setup.py

@@ -32,6 +32,7 @@
         'using different plotting backends.'
     ),
     license='BSD',
+    tests_require=['pytest'],
     entry_points={
         'console_scripts': [
             'rqt_plot = ' + package_name + '.main:main',