fix argument checks in axis/base.margins

If keyword arguments where used, like mentioned in the documentation, the code would fail as
if more than two parameters where given (since len(args) would be 0 -> != 1/2)
removed unnecessary image checks in test_axes.test_margins
added missing cleanup on testbench function
Merge pull request #3078 from andreasWallner/fix_axis_margins_call_args

fix argument checks in axis/base.margins