Add bitrate to animation smoke tests.

This should help catch more problems in commandline argument handling,
like matplotlib#4003.

lib/matplotlib/tests/test_animation.py

@@ -54,7 +54,7 @@ def animate(i):
     F.close()
     anim = animation.FuncAnimation(fig, animate, init_func=init, frames=5)
     try:
-        anim.save(F.name, fps=30, writer=writer)
+        anim.save(F.name, fps=30, writer=writer, bitrate=500)
     except UnicodeDecodeError:
         raise KnownFailureTest("There can be errors in the numpy " +
                                "import stack, " +