Add a new TestCase that asserts that examples exit successfully.

[fischman]

Fixes #909.

Examples changes that were necessary:

• loft.py: failed on macos (only) because of (seemingly) over-precise
  floating-point accuracy assertion. Loosened the tolerance, and
  expressed it as a multiple of the expected value.

  AssertionError: delta=0.002982314711971412 is greater than tolerance=0.001; got=1306.3375467197516, want=1306.3405290344635

• packed_boxes.py: only emit output files when GEN_DOCS is
  True (mimicking lego.py).

[fischman]

@gumyr interested in any feedback you may have on this approach.
Hopefully CI will finish in a few minutes and provide an official take on what's failing and how long the tests take, but in case it's interesting this is what I got on my laptop: gist.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.91%. Comparing base (8e4aa33) to head (80097a9).
Report is 4 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #910      +/-   ##
==========================================
+ Coverage   96.73%   96.91%   +0.18%     
==========================================
  Files          32       32              
  Lines        9434     9434              
==========================================
+ Hits         9126     9143      +17     
+ Misses        308      291      -17     

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[gumyr]

Thank you! The examples won't be forgotten now.