Added mplot3d tests in mpl_toolkits.tests

- This complements PR #2135
- Also tests issue #1933, which was fixed but untested
- Also changes the remove_text decorator,
  to remove text on the z-axis.
Merge pull request #2177 from tbekolay/mplot3dtests

[sprint] Added mplot3d tests in mpl_toolkits.tests