Serialize comparison of multiple baseline images.

[QuLogic]

This prevents the results from stomping on one another, though it doesn't fix the case of multiple tests using the same baseline image.

See #7673 for an explanation. Since there are so many tests with multiple copies of the same baseline (it's not a huge amount, but enough, apparently), this seems like a reasonable workaround.

[QuLogic]

And it died on something else; not sure if that's proof it's working or just bad luck.

[tacaswell]

=================================== FAILURES ===================================
____________________ test_mixedsubplots[0-mixedsubplot-svg] ____________________
[gw0] linux -- Python 3.5.2 /home/travis/build/matplotlib/matplotlib/venv/bin/python
expected = '/home/travis/build/matplotlib/matplotlib/result_images/test_mplot3d/mixedsubplot-expected.svg'
actual = '/home/travis/build/matplotlib/matplotlib/result_images/test_mplot3d/mixedsubplot.svg'
tol = 0
    def raise_on_image_difference(expected, actual, tol):
        __tracebackhide__ = True
    
        err = compare_images(expected, actual, tol, in_decorator=True)
    
        if not os.path.exists(expected):
            raise ImageComparisonFailure('image does not exist: %s' % expected)
    
        if err:
            raise ImageComparisonFailure(
                'images not close: %(actual)s vs. %(expected)s '
>               '(RMS %(rms).3f)' % err)
E           matplotlib.testing.exceptions.ImageComparisonFailure: images not close: /home/travis/build/matplotlib/matplotlib/result_images/test_mplot3d/mixedsubplot_svg.png vs. /home/travis/build/matplotlib/matplotlib/result_images/test_mplot3d/mixedsubplot-expected_svg.png (RMS 0.104)
lib/matplotlib/testing/decorators.py:216: ImageComparisonFailure
== 1 failed, 6221 passed, 9 skipped, 27 xfailed, 3 xpassed in 566.35 seconds ===

That looks like bad-luck, restarted.

[Kojoley]

Yeah, this test fails for all builds from time to time.

[codecov-io]

Current coverage is 62.06% (diff: 100%)

Merging #7674 into master will decrease coverage by 4.51%

@@             master      #7674   diff @@
==========================================
  Files           109        174     +65   
  Lines         46633      56007   +9374   
  Methods           0          0           
  Messages          0          0           
  Branches          0          0           
==========================================
+ Hits          31049      34763   +3714   
- Misses        15584      21244   +5660   
  Partials          0          0           

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