TST: Make sure files are removed in test_graphics

plt.savefig() appends an extension by default, which messes up
ensure_clean(). Added option to ensure_clean() to just return file-like.
Merge pull request #5359 from jtratner/always-cleanup-graphics-files

TST: Make sure files are removed in test_graphics
TST/CLN: Only print on test failure.

Don't pollute test output.
Merge pull request #5360 from jtratner/only-print-conditionally-in-tests

TST/CLN: Only print on test failures.