Close matplotlib figure windows cleanly in tests.

[hodgestar]

Close figure plots explicitly during tests so that the test suite passes when run headless under Xvfb.

Related issues or PRs

• fixes processor plot tests succeed but return error #1639
• see XIO: fatal IO error 0 (Success) on X server ":1001" The-Compiler/pytest-xvfb#11 for the discussion of this situation when using pytest-xvfb

Changelog
Close figure plots explicitly during tests so that the test suite passes when run headless under Xvfb.

[hodgestar]

@BoxiLi Would you be happy to apply this to qutip-qip too?

@drew-parsons Could you check that this works for you and that I haven't missed any other cases? The entire test suite passed for me, but one never knows.

[jakelishman]

Looks good to me, and I'm happy if it works without the nuclear option of pyplot.close('all').

[drew-parsons]

Passes fine for me, patched onto 4.6.2.
Running tests locally,

...
tests/test_processor.py::TestCircuitProcessor::testPlot PASSED           [ 79%]
...
tests/test_wigner.py::test_wigner_clenshaw_sp_iter_dm PASSED             [100%]

=== 1910 passed, 20 skipped, 67 deselected, 53 warnings in 472.83s (0:07:52) ===
autopkgtest [16:47:24]: test test-qutip: -----------------------]
autopkgtest [16:47:24]: test test-qutip:  - - - - - - - - - - results - - - - - - - - - -
test-qutip           PASS
autopkgtest [16:47:24]: @@@@@@@@@@@@@@@@@@@@ summary
test-qutip           PASS

[hodgestar]

@jakelishman @drew-parsons Thanks! Merged and marked for inclusion in 4.6.3.

[BoxiLi]

@hodgestar Thanks for the fix. I'll add it also to qutip-qip.

[coveralls]

Coverage increased (+0.02%) to 65.758% when pulling d08058b on hodgestar:feature/fix-processor-plot-test-under-xvfb-gh1639 into ac27129 on qutip:master.