Add OLED & MiniscreenAssistant tests

[jcapona]

Status	Ticket/Issue
Ready/Hold	Ticket

Main changes

• Added fixtures to use the OLED class in tests.
• Added font files and fixtures to be able to use text-writing methods.
• Added snapshot pytest plugin from pi-top-4-Miniscreen
• Removed Pipfile and Pipfile.lock that were pushed to the repo at some point.

Screenshots (feature, test output, profiling, dev tools etc)

[insert screenshots here]

Other notes (e.g. implementation quirks, edge cases, questions / issues)

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Manual testing tips

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Tag anyone who definitely needs to review or help

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[codecov[bot]]

Codecov Report

Merging #541 (b34e330) into master (c7fc3cb) will increase coverage by 6.42%. The diff coverage is 94.28%.

@@            Coverage Diff             @@
##           master     #541      +/-   ##
==========================================
+ Coverage   50.55%   56.97%   +6.42%     
==========================================
  Files         146      146              
  Lines        6902     6913      +11     
==========================================
+ Hits         3489     3939     +450     
+ Misses       3413     2974     -439     

Flag	Coverage Δ
unittests	56.97% <94.28%> (+6.42%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pitop/miniscreen/oled/oled.py	74.62% <80.00%> (+74.62%)	:arrow_up:
pitop/miniscreen/oled/assistant.py	94.89% <100.00%> (+94.89%)	:arrow_up:
pitop/miniscreen/oled/core/device_controller.py	87.14% <100.00%> (+87.14%)	:arrow_up:
pitop/processing/core/load_models.py	44.89% <0.00%> (-40.82%)	:arrow_down:
pitop/robotics/navigation/core/goal_criteria.py	89.28% <0.00%> (-7.15%)	:arrow_down:
pitop/common/ptdm.py	91.11% <0.00%> (+0.25%)	:arrow_up:
...een/oled/core/contrib/luma/oled/device/__init__.py	20.00% <0.00%> (+20.00%)	:arrow_up:
pitop/miniscreen/oled/core/lock.py	22.85% <0.00%> (+22.85%)	:arrow_up:
...en/oled/core/contrib/luma/core/interface/serial.py	24.65% <0.00%> (+24.65%)	:arrow_up:
pitop/miniscreen/oled/core/fps_regulator.py	25.58% <0.00%> (+25.58%)	:arrow_up:
... and 12 more

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