visual tests: report failed tests

[talaj]

It was missing for a long time that failed visual test names were not reported to the console. This is practical especially on Travis.

 $ test/visual/run --agg -s 1 -j 24                                                                        
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Visual rendering: 3 failed / 353 passed / 0 overwritten / 0 errors

tiff-nodata-edge-rgba FAILED (76902 different pixels)
    "/tmp/mapnik-visual-images/tiff-nodata-edge-rgba-512-512-1.0-agg.png" (actual)
    "test/data-visual/images/tiff-nodata-edge-rgba-512-512-1.0-agg-reference.png" (reference)
gdal-filter-factor-3-lanczos FAILED (101595 different pixels)
    "/tmp/mapnik-visual-images/gdal-filter-factor-3-lanczos-600-400-1.0-agg.png" (actual)
    "test/data-visual/images/gdal-filter-factor-3-lanczos-600-400-1.0-agg-reference.png" (reference)
tiff-nodata-rgba FAILED (156954 different pixels)
    "/tmp/mapnik-visual-images/tiff-nodata-rgba-512-512-1.0-agg.png" (actual)
    "test/data-visual/images/tiff-nodata-rgba-512-512-1.0-agg-reference.png" (reference)

View failure report at "/tmp/mapnik-visual-images/visual-test-results/index.html"