tests/acceptance: Add test of NeXTcube framebuffer using OCR

Add a test of the NeXTcube framebuffer using the Tesseract OCR
engine on a screenshot of the framebuffer device.

The test is very quick:

  $ avocado --show=app,console run tests/acceptance/machine_m68k_nextcube.py
  JOB ID     : 78844a92424cc495bd068c3874d542d1e20f24bc
  JOB LOG    : /home/phil/avocado/job-results/job-2019-08-13T13.16-78844a9/job.log
   (1/3) tests/acceptance/machine_m68k_nextcube.py:NextCubeMachine.test_bootrom_framebuffer_size: PASS (2.16 s)
   (2/3) tests/acceptance/machine_m68k_nextcube.py:NextCubeMachine.test_bootrom_framebuffer_ocr_with_tesseract_v3: -
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  Memgry sackets aea canflqured far 16MB Darlly page made stMs but have 16MB page made stMs )nstalled
  Memgry sackets a and 1 canflqured far 16MB Darlly page made stMs but have 16MB page made stMs )nstalled
  [...]
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  system test raneg Errar egge 51
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  Default pggc devlce nut fauna
  NEXY>I
  PASS (2.64 s)
   (3/3) tests/acceptance/machine_m68k_nextcube.py:NextCubeMachine.test_bootrom_framebuffer_ocr_with_tesseract_v4: SKIP: tesseract v4 OCR tool not available
  RESULTS    : PASS 2 | ERROR 0 | FAIL 0 | SKIP 1 | WARN 0 | INTERRUPT 0 | CANCEL 0
  JOB TIME   : 5.35 s

Documentation on how to install tesseract:
  https://github.com/tesseract-ocr/tesseract/wiki#installation



Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20190813134921.30602-2-philmd@redhat.com>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>